Out of this list of games, which should i buy?

I have the first 3 games on your list and I like them all. Pretty hard to choose between these 3 games. The last 2 I didn't really much care for thats why I don't own them. But if I were have to make a decision I would say to buy in's Creed. Since it looks like you have the desire to play that game the most you should get that one first. Then when you get enough money get GTAIV then Bioshock. If this is for console then this is the order I would choose it in. If you are playing for PC then I would say choose GTAIV first because of the online play. But down the road you should have all 3 of these games in your collection.