Well if it's cannon fodder you're on about - then Barbarian mercenaries and also Spanish and Eastern ones. They're numerous and seldom worth keeping for long. Barbarian mercenaries in particular are too expensive to keep for anything but temporary duties, so sending them over the wall to weaken the defenders whilst my better soldiers wait for the ideal moment to follow is the way to go. Spanish mercenaries are fairly handy in a fight, plus have pila, so they often go first when assaulting a wooden wall (Illyrians are a possible sustitude for these). If no mercenaries are about, it's Hastati or Infantry Auxilia who get the honoured role of leading the assault. They're easily replaced so losing a few is no hardship. Those that survive are typically merged into the next such unit after each battle until they've reached enough experience to be spared and lead the second or third wave. Tis is a nice moment to see your units slowly gain repute and be 'promoted' as reward. Much better than retraining them all the time.

In one such siege where I chose a basic Roman Infantry auxilia unit to lead the assault into the city, I had them enter and go on defensive mode in front of the breach in the wall I had created. I stationed as many archer and missile troops nearby on the other side. The Auxilia unit bravely held off a massive enemy assault lead by a Chosen warlord and several units of infantry and cavalry. After about a minute or more of intense defensive action on my spearmen's part, I sent in the cavalry and routed much of the enemy. The Auxilia had lost nearly half their number, but in front of their spears were the bodies of 150 or more Dacians. Hundreds more lay ahead where the cavalry and then the rest of the infantry had chased the defenders back to the town square. I think the 9+ star general had some influence on the auxilia unit's success, but it was a heroic and sweet moment nevertheless.

Tragically, I have a screenshot of the incident:


What I don't have is the screenshots of the single onager unit killing 292 dumbfounded and chargrilled Dacians before finally running out of ammo and withdrawing.