Krogan soldier, you have an always on 40% (if you focus that way) damage reduction, near 1200 health, and 1500 shields, rage which can boost that up to 70% damage reduction, carnage, which is nasty, and inferno grenades which add up damage fast, and force targets to move or die.
Asari, statis has very good hit detection, alot of power, and with a good group, they utterly nullify the deadly items, especially phantoms, they can lock down one phantom permanently, or keep two mostly down in rotation. (It does take their shields though, so guard your asari) But I havent unlocked that yet.
Turian sentinel, its first, you have a super shield, nearly 1.5x more than your counterparts, innate sniper boosts (nearly 55% stability for little cost, i think 4 ranks in your race special) Which makes widow work very nicely, even the mattock too. The second best carrying capacity, 40 to krogan 50. And of that, 15 innate. Strong natural weapon boosts, near 30% if you want to focus that way. And your armor, which gives 40% reduction as well, so your very capable at range, and still strong up close, especially so with warp or overload. Its all in how your play style works. If you can nab a widow with turian, you are fricken set for one shotting just about anything, save atlas's.