Best Ammo type in ME2?

Seeing as I enjoy using all of them I'm not sure how I'd answer this. I will say I generall yuse Inciendiary Warp an dArmou rpiercing most though but when playing Vanguard I enjoy using Cryo Ammo most as freeze an smash makes for a good combo.

I don't pla yon Insanity difficult yso I don't know.

Depends on the mission. Disruptor is best for synthetics (can stun), Incendiary is best for organics (can cause panic, stop Krogan/Vorcha health regen). Disruptor can also overheat enemy weapons.

The best ammo powers in ME2 are Incendiary, Disruptor, and Cryo since they have crowd-control abilities. I see some people have voted for AP Ammo, but you only use it for the damage, and the damage is actually not as useful as you may think. Ammo power damage is based off of base weapon damage, not total weapon damage. You get better damage from weapons based on distance and defense multipliers. Crowd-control abilities from the big three ammo types make them the most useful.

Warp's extra damage to biotically-lifted enemies is good, but not nearly as useful.

In the end I chose Disruptor Ammo. It is good all around for all enemy types and makes for a good squad ammo power.

Incendiary.

I play on insanity and adapt to each mission (changing squad and/or bonus powers) which is easier when you're playing soldier with all that smorgasbord of ammo powers to choose from. Sometimes you have to go with whatever you've got which is better than nothing.

I'd say Incendiary, with Warp and Disruptor tied for second.

Overload abounds, what with Miranda and Garrus, so an anti-shield ability is not lacking.
Incendiary panics organic enemies and stops their health regen while dealing extra damage to armor.

Unless you're committed to bringing Jacob with you on every mission AND squadding out his Incendiary Ammo, you have no way of replicating or approximating all that.

Best of all, it does all that with just a single point.

Warp Ammo is the most versatile, but has no secondary effects.
You can't go wrong with a 50% damage boost against 3 out of 4 life-meters.
The squad version means only 35%, but it's handy for squadmates like Miranda, Mordin, Kasumi, Samara, Tali, and Legion.

Disruptor Ammo fries shields, overheats weapons, and stuns synthetics.

It's similar to Incendiary Ammo in the sense that it can provide multiple effects simultaneously, but you have to invest to minimum of Rank 3 to get them all.

My most powerful combination I've tried is squadded Disruptor (Zaeed) and Inferno for myself, with Zaeed and Garrus both using the Incisor.
I use a Disruptor-equipped SMG then switch to Inferno-equipped shotgun.

Squad ammos are touchy that way.  It resets if you die and sometimes when you change up weapons.  Frsutrating for sure.  I tend to set all my guns before starting the mission, which allows me to generally change my weapons without affecting the squad's ammo, but the reset still does occur if I die or othewise reload from a save (for example, if I took a break from the game mid-mission).

Ammo powers in ME2 went from being "very important" to "my gameplay loop doesn't give AF about ammo".

Obviously I don't skip them, and I do use them, but I have always used party composition to deal with different defenses so ammo is a secondary/bonus situation, not something I need or count on.

So, I chose Disruptor also. Incendiary is a good choice too, and if I am playing Adept I also like Warp. AP, in my memory, always disappointed me.

For me on Insanity, Shredder and Armour Piercing, everything else can take a hike.

Still pretty surprised by the poll so far, I remember this question has previously come up numerous times in the past including in the original forums, usually AP and Warp are far in the lead ahead of the other ammo types based on Rank 4 for each weapon type:

AP:

Tungsten Ammo
Advanced tungsten carbide rounds in discarding sabots increase the damage done by armor piercing ammo.
Recharge Time: 1.50 seconds
Duration: lasts until overridden by another ammo power.
Armor and Health Damage: +70.00% weapon damage

Squad Armor Piercing Ammo
Time spent custom gunsmithing allows the entire squad to gain the effects of armor piercing ammo.
Recharge Time: 1.50 seconds
Duration: lasts until overridden by another ammo power.
Armor and Health Damage: +50.00% weapon damage

- Good against 2 health types

Warp:

Heavy Warp Ammo
Further increases damage done by warp ammo, which now tears flesh and metal like a miniature disruptor torpedo. Effective against barriers.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power
Damage: 50.00% of weapon damage


Squad Warp Ammo
In an impressive feat of sustained biotic concentration, you can grant the effects of warp ammo to the entire squad.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power
Damage: 35.00% of weapon damage

Effective against Barriers/Armour/Health

- Good against 3 health types

Incendiary

Inferno Ammo
An explosive charge spreads the ammunition's payload on impact, potentially igniting the target and all nearby enemies.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power
Fire Damage: 60.00% of weapon damage over 3.00 seconds
Blast Radius: 3.00 meters

Squad Incendiary Ammo
Each member of your squad gains the effect of your incendiaries, allowing you to spread pain and panic in a very literal crossfire.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power
Fire Damage: 40.00% of weapon damage over 3.00 seconds

Effective against health and armour, the explosive effect of Inferno Ammo can only be triggered when hitting health but the explosive effect of Inferno Ammo may panic secondary enemies through armor/barriers/shields (this only works for the explosive effect - normally, protected enemies won't panic). I believe all synthetic enemies are immune to panic and also some organic enemies like husks, abombs, Klinx

- Good against 2 health types

Disruptor

Heavy Disruptor Ammo
This ammunition's electromagnetic properties can cause synthetics to critically overload and explode.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power
Shield and Synthetic Damage: +60.00% of weapon damage
Synthetic Overload Duration: 3.00 seconds
Weapon Overheat Duration: 6.00 seconds

Squad Disruptor Ammo
Each member of your squad gains the Disruptor Ammo's effects, allowing you to catch synthetics in deadly crossfires.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power
Shield and Synthetic Damage: +40.00% of weapon damage
Synthetic Overload Duration: 3.00 seconds
Weapon Overheat Duration: 6.00 seconds

Disruptor Ammo does bonus damage to all shields and also to the health of synthetics (e.g. mechs and geth) but not to the armor of synthetics (or the armor on other enemy type). The other effects of Disruptor Ammo such as overheating weapons and overloading synthetics only apply once all protection (i.e. shields, barriers, and armor) has been removed; in other words when shooting at the red health bar only.

- Good against 1 health type

Shredder

Improved Shredder Ammo
Your ammo now delivers the ultimate in grievous wounds to fleshy targets.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power.
Health Damage: +80.00% weapon damage

Squad Shredder Ammo
The entire squad now gains the effect of Shredder Ammo, making short work of organic opponents.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power.
Health Damage: +60.00% weapon damage

- Good against 1 health type

Cryo

Improved Cryo Ammo
The apex of cryogenic rounds, this ammunition freezes targets more frequently and for a longer duration.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power.
Freeze Duration: 7.00 seconds

Squad Cryo Ammo
Your entire squad gains the cryo ammo's effects, letting you incapacitate large groups or relentlessly hammer one target.
Recharge Time: 1.50 seconds
Duration: Lasts until overridden by another ammo power.
Freeze Duration: 5.00 seconds

The freezing effect will work on any enemy with a health bar once all other protection layers (shields, barriers, or armor) are removed.

- Good (kind of) against 1 health type

My point being I played most of the game on Insanity with the Mattock and Armour Piercing only, I only ever switched when I unlocked Shredder that I used for the back half of the game, much also a similar situation for the sequel but only Armour Piercing.