Phenomenal game, and now that I see your answer, I'm tempted to change my answer! I'll give honorary mention to KOTOR because I enjoyed it so much, but I think I spent most of my early days grinding away Dungeon Siege 2. I think it was my first rated M game, as well. Good times!KOTOR. Even though it was annoying as hell, and I never actually beat it, I loved it
Mass Effect, but the original trilogy. I like the second one the most. I also love Star Wars: Knights of the Old Republic, it was the first RPG I ever played and I enjoyed it very much. It is a coincidence though that both games come from Bioware but hey, they used to be a good company, now, not that much.
Nobody mentioned Skyrim? well, all of the Elder Scrolls franchise. Skyrim and Oblivion are my favorites. You can play these games every day, all day and still have things to do. You have such a big world to explore, filled with little secrets and things that keep amazing you. Highly customizable characters, awesome variety of classes to play, ...
Just be careful about getting an arrow to your knee.
It's incredible how good Bethesda are at making RPG's (the Fallout series too). A mile above most companies.