What class would you not be able to understand if you were given them all at once? I don't see any class that you would have trouble with for more than one round.
Medic is the best class as far as I can tell for the way I play. The default rifle is one of the best weapons because of it's power and accuracy at long range. Unlocking the blood bag badge allows you to heal yourself fully over and over. The other classes not so rewarding for how I play.
Engineer - The shotgun in the game is decent, but I hardly ever use shotguns in FPS unless I'm running out of ammo. Turrets are too weak to kill anyone by themselves, easily destroyed, too hard to unlock secondary.
Assault - Boost is great, Rocket Launcher is good but only 3 rockets so you have to find a new gun or an ammo station. Not that 3 rockets is wrong, more and the game would become pretty unbalanced, it's just not a class I'd be thrilled about if it wasn't for the boost. Grenade launcher is pretty crappy. For some reason boost does not register in games for me so I couldn't find out about the secondary.
Tactician - Spawn points are good in a team work sense, but sort of useless because the teams can be split into groups of 4, re-spawning with the leader and the fact that the enemy can overtake and camp at the spawn point. Air Support is useless.
Saboteur - Fun to mess around with in enemies base, assassinations or earning some of your other medals (piston, melee, etc) but the starting weapons are far too weak.
Scout - Haven't unlocked but I love sniping in games which is probably why it's the most disappointing leveling system I've played. Forcing you to play for a long time just to unlock one sniping weapon is ridiculous.