What is the worst or most useless power right now?

Honestly I think it would be easier to make a list of things not useless, then fix the rest.

Flak Cannon. Yuck.

And I know that this will be an unpopular opinion, but I don't really like Pull.

Good point, there aren't many powers in the right place right now in comparison to me3.

A lot of powers are really useful but have too long cooldowns, other powers are complete garbage in general.

I personally don't see the point in singularity when annihilation exists, and definitely the assault turret completely sucks with its 24 second cooldown.

Flak Cannon is pretty weak. Definitely one of weakest. I think it should be able to detonate, making the Krogan Mercenary a much better class and the ability more interesting in general. I like Lance on Asari Adept as a second detonator. Cryo Beam would be better if it froze faster. As it stands, you have to expose yourself way too long to be of any use.

Honestly, I'm sorta disappointed by Turbocharge. I've tried it with the Turian Soldier (with Fortify to help with how much you're sticking out of cover) and it doesn't seem to do enough imo. It wants to be both Adrenaline Rush and Marksmen but is worse than either. Maybe I'm not using it with the right weapons though. I've tried the old standards for Marksmen (Charger, Hurricane, Revenant, Piranha) and some new ones (Cyclone, Scattershot) but never seem to get a lot of mileage out of it. Even with Rank 5 Accuracy and Stability, Rank 6 Munitions Training on Turian Soldier, and Rank 10 AR Stock.

I've been getting 1st or second place using the human engineer but his powers are so weak, he's my level 20 lol. Cryo explosions are the weakest power if that counts, flak cannon does more than a cryo explosion... both are terrible tho. I get a ton of assists though, the engineer is a full support class it just needs a boost so I can get more than 15 kills a game. My answer is cryo explosions, my real answer is the mobile cover thingy, it helps in the device objective but then again so does cloak.

"Flak Cannon is raging inferno dumpster tier." This made me actually lol. I'd say lance but at least lance can detonate. Flak cannon is your winner. I don't even bother refilling power cells on it bc its such utter trash. It's almost endearing how bad it is.

My (updated) list:

Flak Cannon should detonate Combos, and/or apply Armor debuff.
Overload chains should Detonate Combos.
Turrets should have lower Cooldowns to allow for strategic redeployment.
Invasion may not be spreading to nearby targets, offhost.

Actually that's all that really irks me. I suppose Cryo Beam should get honourable mention for being such a slow, crappy ability that, despite requiring you to channel it, somehow manages to still do craptacular damage, though it should be noted that it Primes for Cryo Combos before the enemy is completely frozen, so you can do a quick pulse then Detonate.

Flame dot makes Soned and P.A.W. actually somewhat usable (well P.A.W. still needs Incendiary as well).  Considering Turbo Charge's long cooldown, it seems Merc is actually better for consistent dps

assault turret, singularity and shockwave are useless

Sad to see Krogan soldier fucked up pretty bad.His powers seem lackluster.
I play with him on Gold and top score,but it feels more like a struggle.At this moment it is better to go weapon's route.

Assault turrets can be a nice distraction when you do not have really great weapons yet. And a good distraction when you do have good weapons, for that matter.

But, personally, I have not been very impressed with barricade. It has its moments, but it also has plenty of flaws.