Just Cause 2+Hitman Blood Money or GTA 4

if its just the basic gta4 then get just cause and hitman, although hitman isnt that good a game just cause is great fun.
gta4 by itself isnt particularly good but if it has all the dlc then i would pick it because its basically 2 ok games and 1 great game all in 1 bundle. the ballad of gay tony is 1 of the better gta games and easily matches gta san andreas and vice city for mission quality. the other 2 storys of neko bellic and lost and the damed are ok but nothing special. they are both playable and will flesh out the ballad of gay tony and make that part even better but thats the only reason to play em.

just cause 2 is a classic. the game play is truly free. you can do what you want when you want and play the main story as and when you like. its not a particularly good main story but its passable. but its the free for all sandbox nature of the game that i liked.
hitman is as stagnant as its always been. and the last 1 was poor even compared to the other hitman games. so i would say pick something else.

theres so much on sale atm its hard to pick. but valve, thq, paradox and bethista all have good deals... maybe check them out b4 you decide what to but. skyrim is a future classic and well worth the 50% off. and so on.