Isn't "most fun to play" pretty subjective? it's not like, when you enter a MP game, EVERYONE is the same class
Just try them all and find your niche. I know it's definitely not rifleman.
I've said it before, but snipers seem pretty useless to me most of the time. Sure, you snipe someone.... and they just respawn 8 seconds later. Yeah, that's really helping your team win. If this game had rounds, then that's a whole different story, but it's endless respawns so snipers are much less effective and useful. Just my two cents tho.
I'm warming up to assault, and I've seen some *fantastic* assault players, but I use it just to unlock the boost. I'm not good with the class, and I drop my grenade gun the second I find any rifle.
Engineer is a pretty good class. Well-placed turrets, while weak, can be annoying and a good initial deterrent. The shotgun isn't as overpowered as it used to be - miss once, and a good player will punish. One shot will do a lot of people in, though. Just watch out for other engineers on public servers destroying your turrets so they can place theirs. Annoying.
Medic - I wish we had more medics. Such a useful class with the blood packs unlocked. Being able to heal yourself whenever you want is pretty handy, and you start with a rifle, so it's a pretty good class.
Tacticians are important but, again, you are fighting against dumb teammates to correctly place the 2 spawns.
And saboteur is the class I settled into. I love the SMG, the C4, and watching the enemy run right past me. Some people find the class pretty difficult to use, though.
Just give each class a couple hours and dont worry so much about it. Learning the maps like someone else said is far more important. I SUCKED at the game for the first couple of days, then it just clicked, to use a cliche. Just remember that you can't "spray and pray".