[Accuser]

Tanks:
Magrider
Prowler
Vanguard
This is probably a biased list. The Magrider is spectacular at what it does: killing enemy vehicles (and aircraft). If that's your measure of a tank, then the Mag wins hands down. However, a Prowler does well against vehicles and infantry AND is best able to breach a courtyard so that friendlies can push in. So I could see it tying for first if you valued this as well. The Vanguard just isn't the best on either front.

AI Max
Dual-Cycler / Scatmax
Quasar
TR and NC tie for first and really, each is great at its own role (though the Quasar's TTK is far too low). A locked-down Dual-Cycler probably has the lowest TTK of anything in the game and is glorious on defense, probably my favorite to use. If an enemy pokes his head around a corner to look, that head is coming off.
Scatmax is best for attacking indoors, obviously.
And with the Quasar... you're defending safely from the cover of the base wall, until suddenly you have a squad of VS Maxes jumping on your face. They're great for claiming a courtyard, but the long TTK makes them SUCK indoors... which is exactly where the VS had the most problems in PS1.