Hmmm, well I like them all, and I mix it up as they are all fun to play.
I would say the Assault class purely because the XM8 is awesome, you can get points from ammo supplies to team mates, and you have a grenade launcher.
Then again, the LMGs that the medic has are also pretty accurate if fired in bursts, and you have the benefit of bigger clips, and massive points from heals and revives
I was just about to start saying how the Engineer is equally as good, but I won't, as I'll end up listing the good things about recon as well. The point I'm trying to make is that the game is extremely well balanced in my opinion.