You should definitely try the older ones. At least San Andreas. It's pretty detailed and varied too. All cities and counties have a distinct feel, it doesn't feel or look the same at all. I loved III, VC, and SA, but I love Rockstars newer games as well. Except L.A Noire. What the hell was that?

You might like Saints Row or Just Cause if you liked the older GTA's. I feel that those are more like sequels to the older GTAs. But beware, they're obscenely over the top.
Yeah, I might try in the future San Andreas but with the coming of GTA V, who knows. I'm aware that San Andreas is to GTA as Morrowind was to TES. If I google which GTA was best almost 80% say SA.
Btw, I've Just Cause 2 and that game is amazing. Even though I like simulator games(like Arma 2) JC2 is a blast to play. What's remarkable about that game is the cheer size of the game map. About 400 square km of sandbox gameplay and without loading screens or stutter. JC2 is without a doubt one of the most stable games I've played. The graphics/shaders, even though they're about 2 years old, they are in many instances more sophisticated than Skyrim. The LODs are done very efficiently and you can hardly see any pop ups. In fact, in certain areas the textures looks Crysis like. How in the hell did the devs managed to create such a detailed massive game world is beyond me. The Avalanche Engine must have some serious tech there.
I consider JC2 like a playground, like Universal Studios where you can do just about anything, go anywhere anytime at your own leisurely time. Some people say it gets repetitive but for me there is A LOT of stuff to do and collect all over the huge map. Btw, JC2 map is over 14 times bigger than Skyrim.