Sentinel. I enjoy playing with both Biotics and Tech, and it gives me some of the best of both worlds, plus in 2 and 3 solid enough combat skills. In 1, on a play through after the first a Sentinel with a Sniper Rifle or an Assault Rifle could hold up quite well too, and even without the better weapons, was still totally survivable. The ability to strip all defenses, while maintaining solid defenses of your own is quite nice. I'll admit I rather missed the loss of the Medic option from 1, since I enjoyed playing that role, but... I can understand where 'The Hero of the Citadel' is unlikely to be a combat medic as their primary focus, and can see thematic reasons to choose to drop it...though I still enjoyed playing as such.
I also enjoyed Vanguard, though not as much. Soldier was...somewhat meh for me, though in 2 it was much more enjoyable than in 1. In 1...it felt like I had no real reason to bother playing Soldier, since I could just as easily be something else and be as good at the same thing anyhow...'Yay, I shoot guns good...so does everyone else...congrats'. Adrenaline Rush and the Ammo Powers made things a bit more interesting there in 2, though I still found it less interesting to play than Sentinel or Vanguard. Adept... mostly felt like I didn't really need that many Biotic powers, when I could do just fine with Sentinel or Vanguard...and I tend to prefer multi focus characters anyhow. Engineer and Infiltrator just never gelled for me... I tried them, but found their play style uninteresting. Engineer wasn't that bad, but... I mostly felt like a Sentinel that forgot how to Warp. The lack of the same level of defense stripping and the lack of the whole 'Gravity is my friend and not yours' deal just made it less interesting. Plus, I already had Tali filling a similar Tech primary role there often, or someone else if she wasn't available.