The Soldier seems the best to me, because canon Shepard is a soldier. So it seems logical to do what canon Shepard does. Going canonical, so to speak. I really like writing "canon", because you see so many fucktards online spelling it "cannon" while talking about Star Trek or Star Wars, it's really annoying. So it's empowered to spell it "canon" because that's how it's supposed to be spelled. Canon.
Also, the Soldier gets to use the assault rifle, which is probably the best gun in the game. Ya know, it fires really rapidly, has a pretty good range, and looks nice. Good all-rounder.
Lastly, the Soldier from ME1 had Immunity, the best ability ever. It pretty much made you invulnerable. I really enjoyed using Immunity and soaking up tons of damage. Now there doesn't seem to be any Immunity in ME2. But I'm gonna go and find out if that is indeed the case. Maybe we're wrong and Immunity is in ME2. Or maybe it got changed into another ability and actually got an improvement. Or maybe it's gone and I'll find out and get really pissed off and write up my thoughts on its loss. So we'll see. We'll see indeed.
BTW, they showed the first 12 minutes of ME2 on Gamespot. Apparently, you'll be able to collect spaceship models to display on a wall in your ready room. Spaceship models are awesome. Cause in a tense dramatic moment, Shepard could get really pissed off and throw his assault rifle around, knocking the spaceship models to the ground. And then Miranda Lawson will reach down and pick one up and go "You broke your little ships."