In my opinion there isn't a single RPG out there that ticks all the boxes, i.e. great combat mechanics, character development, story, a great game world to explore, etc.
The Elder Scrolls certainly nails the game world and for my part character development and story too (the main story is negligible, it's all the little quest story lines that make it so good), but seriously drops the ball in terms of combat. A game like Kingdoms of Amalur nails the combat and character development, but its story line leaves a lot to be desired. The Witcher 2 nails the story line and combat (though it's love it or hate it), but doesn't have a lot in terms of character development. Mass Effect (the original) does a lot of things right, but outside of the storied locations, the game world becomes incredibly generic and boring to explore.
The perfect RPG would need to be a lot of Elder Scrolls with Kingdoms of Amalur quality combat and a story line and characters as strong as in The Witcher 2. I haven't seen it yet. Perhaps it's better if that game never exists, as it would probably be the end of my social life.