IMHO, i rank it like this.
Anti-Tank is on top, since the class gets the most utility. It has body armor, a decent weapon, and the Rockets allow it to tango with armor and infantry, as well as allowing it to snipe.
Following Anti-Tank is Medic. Medic is probably the most popular class, being the class of choice for most point whor....i mean top players. On a serious note, Medic is a really bang up class, provided that the player isn't out to whore points.
Now, the next two classes fluctuate, depending on if its a vehicle map or infantry map.
If Infantry, it goes Support, then Engineer. If its Vehicle, it goes Engineer then Support.
Then Spec Ops, mostly cause the G36C is hella accurate and the C4 is pretty useful.
Assault is next, leaving way more to be desired in the way of utility, but you gotta have your basic grunts i guess.
Sniper is in dead last of course, since...well they really don't do much. Your primary function is recon, and thats moot if you have a good commander, who actually spots after sat scans and zooms in on the map to spot outside of the UAV's area.