can't decide on a build

Thanks for the tag, dazk . You know how I like these discussions.

lynx7386 , my current build ticks a lot of your boxes. I'm playing a Sentinel (Lance/Annihilation/Energy Drain). I like to call him the BioTech Bomber. I'll explain why I think it's awesome both in gameplay and in terms of RP. You've chosen "Leader", so you're already headed down this path. Sentinel, Energy Drain and Annihilation are already open to you by design. Lance perfectly fits the mix since Sentinel was tailor-made for it. 

RP Advantages: This is easy. Sentinels are fun because they're techie biotic soldiers. They do it all! There's no RP situation in which you'll think, "Would my Ryder really be conversant on this topic?" Talking tech with an Angara? No problem. Talking Biotics with your Asari/Huntress waifu? No problem. Guns? You use those well, too, and have customized some real killers. No skill or approach is too far outside the Sentinel's likely area of expertise. This build combines the Tech of Energy Drain and flash-forged Tech Armor (cool-looking and effective) with badass Biotic skills Lance and Annihilation. 

Gameplay: This could get long, so I'll try to build a reasonable list of all the great synergies in this build.

Sentinel Profile: You get the absolute perfect perks for this build! 
* Combo Damage: This is the only Profile with a straight combo damage increase. You're also going to craft gear to build upon this.
* Tech Recharge Speed: Useless, mostly, since you're going to get the Fusion Mod of Adrenaline on Havarl. It resets cooldowns every time you kill a foe in any fashion. Glorious!
* Power Shield Cost Reduction: Could be called "Lance Cost Reduction". You'll evolve Lance to run off of your barrier, rather than a cooldown. (You may've done this with Nova, previously). Between this perk, your passive skill evolutions and the gear you'll use, Lance becomes very "inexpensive" in terms of shields.
* Power Restoration and Defense: I'd wager most players don't know what this does. It improves the effectiveness of skills and effects that restore your shields. This is awesome, since you'll be in a cycle of using restorative Energy Drain and destructive Lance to prime and detonate foes.

Explorer: It's not as perfect as Sentinel, but Explorer is still great for this build. You lose Tech Armor, but gain lesser damage reduction and Biotic Blink. Most importantly, you retain PR&D. The biggest loss is the Power Shield Cost Reduction. Your Lances will be notably more costly without it. You do get a solid weapon damage boost.

Skills

Annihilation: You've used it, so maybe you know how awesome it is. Build for radius (4A?) and damage attractor (5A?). I prefer 6B Vortex over the more supportive 6A evolution, since you'll have more than enough shield regen happening. Vortex makes ambushing wraiths and charging foes a triviality by lifting them into the air. 
Lance: The exact evolution you choose is up to personal choice. I actually go for radius and dmg vs shields, since I'm on PS4 and this better fits that controller (and the game's hidden headshot workings). You definitely want 6B, making Lance run on barrier in lieu of cooldown. This allows you to throw that extra Lance or two if a combo doesn't kill everything. It's great against bosses who won't be triggering your FMoA. Plus, it's lore- and logic-friendly. Siphoning a little barrier to form a projectile just makes sense.
Energy Drain: It's base damage is unimportant, so evolve for AoE, support-effects and Prime/Detonate duality. This power, combined with your Tech Passives, is going to make you immortal. You'll constantly be priming, detonating, healing and regenerating shields.

Tech Passives: These are the most powerful skills in the game, but are often overlooked by many players.
* Team Support 5A and 6B go a long way toward making your entire team unkillable. 6B, Life Support, heals you for 3 sec after using a tech power. You'll be using ED a lot.
* Offensive Tech improves your detonations and gives you 6B Technical Rounds, which improves weapon damage by +30% for 5 sec after using a tech skill. Again, ED a lot.
* Auxiliary Systems improved priming rates and radiuses. 6A Shield Feedback starts your shields recharging immediately after using a tech skill. See a pattern?
With these three, you'll be constantly regenerating health and shields because of Energy Drain. Sentinel's PR&D and the gear you'll use make this even better.

Biotic Passives: You'll want all of these, too, naturally. Always go for radius, combo damage and more utilitarian boosts over niche benefits or recharge (unnecessary, given FMoA.) I can give specific advice if desired.

Combat Passives: You'll eventually want all of these, too. Start with your preferred weapon training and Combat Fitness, then expand from that point.

Weapons

You'll likely want a custom Valkyrie or Shotgun. The Valkyrie is nice because it's long range. Even though this build is invincible up close, it's still great at range. An AR helps with that. If you love or prefer shotguns, I can suggest several insanely powerful, fun designs. The Valkyrie I suggest is this:
Valkyrie (Automatic Fire; Bioconverter; Dual Mod Extension; Kinetic Coils x2) with Barrel, Clip, Receiver and Stock mods.
This turns the vanilla Valkyrie into a fully auto, very accurate fountain of death. It's the only gun you'll ever need.

Melee is important, too. It's useful in its own right, but most importantly it gives you 4/5 augmentation slots for "always on" boosts to your character. I'd recommend the following:
Asari Sword (5 Reaction Optimizers): This pairs the best melee weapon in the game (complete with frames of invincibility via that thematically appropriate biotic slash) with +12-15% Combo Damage. Nice! Between Sentinel, this and your armor, your combo damage is really boosted.
Alternatively, you could use the Remnant Cryogauntlet. It's also an Ultra Rare with the augmentation slots you want, but isn't quite as great as the Asari Sword.

Armor

The game is easy enough that you can likely use whatever you want. That said, Pathfinder Armor is great for this. It's advantages are as follows:
* No R&D cost: You get the blueprints for free, allowing you to focus R&D on your gun and sword.
* Damage Resistance: Who doesn't want this?! (Tann voice)
* Weapon Accuracy: Universally useful, especially with an autofire AR.
* Power Restoration and Defense: Just like the Sentinel Profile! This makes your ED and Tech Passives even more useful, and pushes your closer to invincibility.
* It looks badass. Seriously, it's awesome looking armor.

Armor Augmentations: This is pretty simple.
*Chest: Every chest piece you ever make should contain a Shield Oscillator. It restores +25% of shields when you kill a foe. Perfect for anyone, but especially for you.
* Legs: I like to use one slot to add elemental damage (electrical, cold or fire) to my jump melee, since I like to use melee pretty often. This is up to personal choice. An extra Reaction Optimizer might be more optimal.
* Everything else in every slot: Reaction Optimizers to boost Combo Damage. You'll be destroying most of your foes via combos, and this gives you that extra damage to ensure few survivors after your initial onslaught.

Fusion Mod of Adrenaline: This is a crucial piece of gear for any combo caster build, but especially ultra aggressive builds like this one. If you don't know where it is, I'm sure we can find and link a vid. It's found on Havarl via an unmarked Remnant Puzzle. This will allow you to tread upon your enemies, hurling death (in the form of ED and Lance) as you like. Cooldowns are for mortals. You are a BioTech god.

Am I forgetting anything? It's late and I'm sleepy, but I think this covers most bases. Ask questions as needed, and I'll clarify anything, give specific advice, share my experiences, etc... I'll say now that I wade through my foes in a non-stop storm of Combo Detonations, melee attacks and gunfire with this build. It's one of the most powerful possible builds in the game, but isn't as prone to staleness as the Charging Vanguard. I can attack and combo at range, up close, whatever I like.

EDIT: Tomorrow, after a proper night's sleep, I can list the build I use (if the interest exists) in the X: 4A, 5B, 6A style.

Some other questions:

Why use electrical conduits on the reegar? Doesn't it have that functionality already?

I'm a fan of the x5 ghost, like a better avenger, but what mods work well on it?

Also thinking maybe an elementalist set up on this sentinel would be fun: singularity for biotic combos, overload or energy drain for tech, incinerate or flamethrower for fire, and remnant gauntlet for cryo. Thoughts?

In the event I get to a new game plus after this play through, I'm planning on doing another run with Sara. I want to know how well a non - vanguard melee build can work; w thinking of combining explorer profile's biotic blink with the asari sword for a teleporting melee build that's all over the place, but I'm not sure melee is all that viable as a main source of damage without vanguard damage and shield bonuses.
Was thinking something like asari sword, explorer for biotic blink, tactical cloak, and maybe like grenades, invasion, or a turret for cover fire with cryo ammo?

Still wish there were more than 3 ability slots at a time, I feel like my options would be far less limited with four abilities active.

Element zero I tried your drain/annihilation/lance build and tbh I'm not a fan. Its mostly to do with lance, though - for some reason I have issues reliably hitting with it and the shield powered bit has gotten me killed twice now and often my shields are out and I'm waiting on energy drain to cool down and can't use lance without any shields. Lance seems particularly hard to use up close for some reason on faster moving enemies (havent unlocked vortex yet).

I'm thinking about going with drain, annihilation, and overload or incinerate instead. I've enjoyed doing multi combos with the current build (annihilation prime, lance, melee prime with cryo gauntlet, lance, energy drain to tech prime, lance again, three combos in a row). With incinerate instead I could do all four combo types as needed.

Also I'm still deciding on armor and weapons. Been using pathfinder but I might try Heleus defender or angaran armor instead. Also considering using an ushior with the augment that boosts power damage by 20% when your clip is full, as it would be active all the time with the 1 round magazine.

Yay its not just me, I struggled with Lance too 

I just think I prefer tech over biotic abilities, habit I guess as I've always played an engineer or infiltrator main in past games.

I'm still working on getting the adrenaline mod, but after that things should go better. Annihilation seems useless except against charging dogs, as I more often play at range and use cover. Maybe I should just go energy drain, overload, and incinerate for endless combos?

Patience is not my strong suit!!!!  I wasn't too bad with the aim, I got sick of Kett etc doing there little jump sideways thing and not being able to anticipate it.

So have been running CC/ED/ZAP which has been fun but going to dump ZAP its having spawning issues in that the bloody thing won't go away, ever. Its even appearing on the Tempest all the time. Trying not to go back to my soldier build, will have a look and try and find something I haven't used.

CC - Concussive Shot, I can't give it up it gives me too much pleasure detonating things with it!!!!! 

No real plan, been playing as a Sentinel but switched back to Soldier for a bit yesterday to see what difference it made in combat. TTK (Time to Kill) was definitely shorter but not a game changer so happy to switch to Sentinel again or something else. Make a suggestion and I'll run it.

For now will change back to shockwave that primes doesn't it?