GTA 5 is probably the best game I ever played. I liked the story and city better in GTA IV though, but the updated graphics for V are pretty incredible. Either way, you cannot go wrong with these two games. You will have hours upon hours of enjoyment and it sure beats spending money movies nowadays.
My only complaint with GTA V is that it is too short. You get really into this game and start to love the characters. More missions and a much longer campaign would have been awesome. They could have doubled the length of this game and I would not have complained.
There arent too many must play games out there, but this and GTA IV are one of them.