Grand Theft Auto San Andreas is most likely my favourite game of al time.
I've lost count of how many hours I've spent in the multiplayer platform "Multi Theft Auto". It's pretty amazing, especially roleplay servers.

If I were to mention an original game, with no modding needed, I'd probably go with Grand Theft Auto V, and I'm sure, when Grand Theft Auto VI is out, that's going to be my new all-time favourite.