Xbows don't shoot as fast. Thus, Venetian archers can kill enemy units faster. Once they fix the passive AI, xbows will not get ability to expend all their ammo.

There is indeed no "best" stack. I assume this is for SP. If so, then exploit the passive AI all you want and load up on Xbows and Venetian archers. Throwing in a piece of siege might also be good. Then add in 4-6 units of heavy cav to break up the lines and rout chase. That's for a general AI stack.

Some bread and butter units are Italian spear militia and Venetian Heavy Inf for anti-cav and anti-armor respectively. Carraccio standard is likely a waste unless you have tons of militia units.

Note that catapults are a must for bridge defense as they can shred enemy troops on the bridge.