Nearly all of them are strong Melee users.
The Female Turian is essentially Wolverine with poison as her powerset focuses on melee attacks with poisonous claws. Her heavy Melee teleports to the target and starts a combo where additional attacks teleport her around them slashing. Her 'charge' has a short distance but everything from the start of the charge to the end of it ends up poisoned. She makes quick work of Ravagers and swarmers with little effort. One of her powers is a shield/durability boost for a period of time and if specced right, can give a half second of invincibility. I've yet to find the optimum use of her Neurotoxin blades but they do decent damage.
Alliance Infiltration Unit has the fastest heavy melee attack, can self rez as long as she dies with her 3rd power active, and gets shotgun damage bonuses. Give her a Pirahna Shotgun and you can stealth up to your target, unload a clip for massive damage, possibly heavy melee, then bail/repeat as needed.
Talon Mercenary... well I've yet to figure out how to use him well. The trip mines are crazy powerful but his armor piercing arrows, concussion arrows, and Omni Bow in general seem kinda subpar. That they all use the same resource is annoying as well.
Krogan Warlord= Melee all the things. Switch between charging your hammer with electricity and biotic power as needed against the opossition. You have a ton of shields plus the Sentinel armor power on top of an insane amount of health regen once you melee 3 people. You will be having fist fights with Atlases and I think they're big enough to be immune to sync kills like the Juggernaut.
Awakened Collector: Haven't unlocked yet despite blowing 2 million creds. I'm wondering if he's an ultra-rare.
I did unlock the Geth Spitfire assault rifle last night though. It's basically a rifle version of the Geth MG which means it makes an annoying sound but sends an insane spray of bullets that gets faster the longer the hold the trigger. Tried it on a Turian Ghost Infiltrator and was able to stealth multiple times in the same clip for bursts of damage. Great for stripping enemies of their barriers and generally softening up a crowd for the powers of your allies. I'm gonna try it with stability an accuracy mods tonight and maybe on a Soldier with Marksman. So far the only downside is that its so heavy it actually has the added effect of slowing down your walk speed.
Still missing the Gear items, Lancer, Venom Shotgun, Harrier, Typhoon and Awakened Collector.