I have played all three of those games.
Protoype : Complete attempt at a copy of Assassins creed, although failer miserably IMO, fighting is Soooo repetitive, and the game play is not fun at all for me. I played 2 hours of it, and had to stop becausei hated the game. The controls were also very clunky and the game was basically a button masher when you attempt to get combo's off. Over all the game could have been good but the game fell far short. Storyline was also pretty lame.
GTA4: Fun, but same old thing as the GTA series. Not really much more to add then that. I found myself in this game not wanting to do the storyline what-so-ever. I just stole cars, bouth stuff, and ran around doing my own thing. In vice city (my favourite GTA game) this was sort of the case, but i also wanted to complete the main story line and complete the game that was designed. Not just randomly run around doing nothing. The graphics in GTA 4 were great though, much better then vice city, although being able to fly out the windsheild was kinda annoying.
Assassins Creed: Great storyline that you really want to know what happens, some say the assassinations were to similar, but i loved them. The only gripe i have about the game is the ways you get information are all the same, and having to scale lookout after lookout. Graphics are amazing, i like them much better then crysis. Voice acting was doog as well. Fight scenes were an absolute blast, and the rush of excaping from killing your target was the most fun i have had in a game like that in years. Over all a great game other then the repetitivness that i mentioned.
So out of those three games the ranking IMO comes in at:
1) Assassins Creed 2
2) GTA 4
3) Prototype (dont even bother with this game)
If you ever want to play Assassins creed 2 in the future, which BTW releases tomorrow in NA, i suggest you play Assassins Creed 1 to familiarize yourself with the style of the game, and learn the storyline behind it. If you just jump right into Assassins Creed 2 alot of things that were explained in the frist game will be pretty confusing. It is deffinitely worth the time to play Assassins Creed 1.
A thing that i am really looking forward to in AC2 is the economic system, that would have made the frist game golden. The ability to swim is also a good bonus lol. In the first game you got this guy that is an elite assassin and if he falls in water he drowns to death with light leather armour on lol.