Assault is obviously a good choice as being the all rounder...
I used to use the M416 a lot but changed to AN-94. Head shots with this are as easy as stealing a baby's pacifier, if the red dot scope is on
Medics are fun because you can make the opponents crazy with getting killed by those who they just sent to sleep / m60 or 870 combat (for fun) I normally use
I've been playing engineer mostly until i got sick of running after tanks
(ump45 or pp2000)
as always checking what class the rest of the squad has should be first choice anyway