Define best

The class that's the strongest and most fun to play

I think Sentinel is the one that fits both categories the best, Infiltrator is strong as hell if you get the best Sniper Rifle but some people might find it kinda boring, Vanguard has a lot of cool powers that make it fun, but it's not great against certain enemies and on harder difficulties, Adept is pretty much the same as Vanguard but a little different, i heard Soldier is also strong but idk how fun it is in ME2.

The 'best class' is the one that best suits your personal playstyle. The happier you are with the class abilities will correspondingly make that version of Shepard 'the strongest and most fun to play' for you. If you like Combat, go Soldier, Vanguard (with Biotics), or Infiltrator (with Tech). If you like Tech, go Engineer, Sentinel (with Biotics), or Infiltrator. If you like Biotics, go Adept, or one of the other two crossovers (Sentinel or Vanguard).

Like what you play and play what you like.

That's a good point, some of the classes have things that are objectively better, but if you don't like the powers and weapons you're using you'll probably make more mistakes and that class being the "strongest" won't really mean anything, but most important you won't have much fun with it.