If you've never played a GTA, get that. If you have, I'd reccomend Brink. Brink has a lag fixing patch coming out either this week or the next and will have free DLC coming out in June. And if for whatever reason you want to get Sniper: Ghost Warrior,
get a trial if there is one or rent it before you buy it. I've heard it is god awful.
Easy GTA 4. It's the better game. Brink is OK, but it has a better multiplayer than single player. And Sniper is just not a good game. Only get it if you can find it super cheap and have nothing else to play.