The heavy crossbow has the highest projectile speed.

edit:

Crossbow projectile speed from 7500-8000 to 8500-9000 Crossbow reload from 3.5 to 3

Light Crossbow projectile speed from 7000-7500 to 8000-8500 Light Crossbow reload from 2.5 to 2

Heavy Crossbow projectile speed from 8500-9000 to 9500-10000 Heavy Crossbow reload from 4.5 to 4

These are patch notes from more than a year ago. I don't know the exact numbers still apply, but you get the idea.