Seriously? Grand Theft Auto is the best series ever? Sure, you get to kill, steal, and get to have sex with different females without any consequences, but that isn't what a great series is made of.
A great series consists of two things, and two things only: awesome gameplay, and a kickass storyline.
I could name a few series right off the bat that are way better than GTA. (NOTE: the things I list are either a single game or a series, so don't get confused if you look one up and see that it only had no sequel.)
1. Final Fantasy
2. Okami
3. Ninja Gaiden
4. KotOR
5. Destroy All Humans!
6. Zelda
7. Alien Homonid
8. God of War
And that's the only ones I was thinking off of the top of my head. Okay, GTA may be a good series, but it certainly isn't the best and many people will agree with me.
I guess it's just a state of opinion really.