Someone's earlier post picked it right, some people are still thinking what their favourite kit is.
The question should be "... to use PROPERLY".
Ignore morons grenade spammers as support, or being super assault as medics and never healing or reviving others. Assume people are playing the game "properly".
It is from medics, spec ops, AT or engineer.
There is nothing more inspiring than seeing a medic playing an awesome game, racing around reviving buddies and keeping an attacke going. How they survive when every one is trying to get them is a miracle in some games.
Support is tough, because the guns suck, and keeping everyone supplied is unglamourous and boring. But the number of times I have run out of AT rockets, or ammo, and wished someone was support nearby, has literally killed me.
Spec ops on fire are awesome tank killers. Sure assets can't fight back, but it takes patience and balls sometimes to creep in and blow some of them up when they are properly defended. All for one point. But nothing gives more satisfaction, or annoyance when in the tank, than spec ops plopping a C4 on the tank rolling by (especially in non-urban). Or seeing vehicles crossing the Lake in Jalalabd getting fried because some specop left a present under water and was watching ... hehe
Engineers are tough played properly, because like support they get little reward for a lot of unsexy work. The shotties is great in close, but useless elsewhere. The AT mines are great when used properly, but spammed too often can TK or block proper access to a painful level.
AT? Having played it far too much early on when I was learning, the SMGs are crap. Assault kit is awesome after getting massacred by everyone else on the map. But going head to head on your own with a tank, and knocking it out, is the most awesome feeling in BF2. Apart from shooting down choppers with AT ....
So my vote is for AT. When you are infantry and the tanks roll in, and there isn't any urban warfare for the specops to exploit, you're always grateful to see those missiles screaming in on the bad guys' armour.