I liked GTA4 out of all 3, and I've played all of them. Both Prototype and Assassin's Creed got very repetitive, and with that, boring. GTA4 = MODS, MODS, MODS, and MORE MODS!!! Even with the game at stock, it's still better than the others, 'cause not everything is the same. Assassin's Creed, you ride the horses, kill the guards messing with the citizens, fight the "bosses", then travel by horse back again... Prototype, you get a few different fight moves, lose them, run up some buildings, kill military, and infected, fight the bosses, and gain your moves back as you progress.
GTA 4
Prototype
Assassin's Creed
In that order.