Most of Roman infantry can be used as assault units. I personally like Equites Extrodinarii, heaviest Roman Pre-Marian cavalry. Thessalian Hippies are your next best bet, since they're rather hard to obtain in most places. Beyond that, you can also use Hellenic Peltastoi, their role was exactly that, but was meant to be used in support of Phalnax formations. They work well with any reliable heavy infantry, however.

The only times I use other units is when on an extended campaign and unable to reinforce the armies with proper Roman forces. Admittedly this has turned from "rarely" into "all the time", due to Roman conscription limitations.

Oh, and by the way, (useful) frontal cavalry assaults are about 400 years away when the game starts, and some 800 years away from true effectiveness. The cavalry goes into enemy flanks and rear, if at all possible.

McHrozni