Asari 
I've said it many times, I'll probably say it again... Valkyrie is simply the best. The combination of tech armor, annihilation field and warp makes her a powerhouse, be it on defensive or offensive. Put adrenaline mod on her and Raider and watch her melt everything in front of her while she still manages to stay sexy. Piranha is a good alternative.
Asariguard is probably my second favourite, despite not hosting often. Them lift nades... sure, they are not cluster nades, but they still manage to bog pretty much everything down. And watching phantoms and nemeses flying into the sky never gets old. Yeah, she doesn't have much in ways of killing armor, except for her guns. Which just serves to show how badass this gal is... she does everything her sisters do and without fancy magics. Only her and Claymore and enough ordinance to blow Harbinger back to dark space.
I like Huntress. She's that creepy sneaky ****** that will sneak up on you, infect you with space plague, blow your head away with Claymore and then blow up your buddy with a well placed warp that will also blow away half of your unit, all the while laughing maniacally. Seriously, she's kinda creepy, but so very much effective.
Vanilla adept was my first love and I still love her dearly. Dem biotic booms... dat stasis and all dem blown noggins... beautiful! Lately, she's gotten a taste for snipers, like Kishock and Krysae... blowing up Cerberus grunts into gory pieces never gets old.
Justicar... don't really care for her, since I suck with her. I know how incredibly powerful she can be, but I just can't get a proper hang of her.
And since it needs to be said again...

