I'm usually sharp, but when I have a growing team I pick whatever we're lacking to help balance our abilities out. I'm level 5+ in basically everything. Only thing I don't play a lot of is medic.
And no, I don't use the M14, ever, for two reasons: it means I can't carry a crossbow, which is the weapon that exemplifies the class role, and it's less effective as a battle rifle than the SCAR, so if the team needs a guy with a battle rifle, I just switch to commando.
in order, my fav classes are:
Sharpshooter (for taking down fleshpounds/scrakes on Hard or Suicidal difficulty)
Support (for additional carryweight/ability to carry an AA12, an M32 and either a katana or pipe bombs, and for holding doorways)
Commando (if the team doesn't have a commando with a microphone, they need one, and I'll play as one, especially on levels like Farm or Bedlam. Very effective for keeping a team afloat if everyone sucks too much to take down crawlers and the like)
Demolitions (I tend to play when I have a team full of commandos and sharpshooters, or just feel like blowing **** up. Good for killing patriarch.)
Bezerker (good class for starting the first couple waves and conserving cash, use no ammo, keep armor if you have any left, and sell your weapon when moving up to another class)
Medic (Not /quite/ my least favorite class... the extra running speed means that if my team -really- sucks, I can usually pull a monty python and finish a round on my own. Always carry an xbow and a katana--MP7 is for chumps)
Firebug (I hate firebug, I really do... it's the one class that really depends on the rest of the team to finish things off, because there's no stopping power or decap to the flamethrower)