If you aren't too fussy about a few glitches here and there, then The Elder Scrolls series from The Elder Scrolls III: Morrowind up to The Elder Scrolls V: Skyrim are all really good games from experience, what my friends have said and what I have seem of the games being playing. I personally play The Elder Scrolls IV: Oblivion, as it is the only one of the three I personally own but I think I'd enjoy it more than Morrowind anyway. It's a great game in my opinion, and a lot of the annoying glitches are removed in the Game of the Year Edition and any that are left over are fixed out by the Unofficial Patch that was created by the community. Another set of RPGs created by Bethesda Studios, the same company that made The Elder Scrolls, has also created Fallout. I'm not sure about the first two games, but I think that Fallout 3 and Fallout: New Vegas are both really good. A lot of the DLC for all of these games is very enjoyable too, I've only come across one DLC on Oblivion which I dislike, it's a Horse Armor DLC. In the game the DLC is completely useless, so it doesn't really do anything to improve the quality of the game.
I've not played any of them, but I have heard that The Knight of the Old Republic games are all excellent games, and some friends have said that the Mass Effect series and Dragon Age series are both very good as well.
I personally prefer The Elder Scrolls games over the other ones that I have mentioned. I'm not sure if that is because I tend to prefer sword fighting style games over games that are FPS/shooting games. An exception to that is Borderlands, it's not a full RPG, but it does have quite a few RPG elements about it. It definitely ranks as one of my favourite games, but in my eyes the DLC doesn't really become good until The Secret of General Knoxx and Claptrap's New Robot Revolution were released. If you do buy Borderlands, then I suggest that you go for those DLC first or just get the Game of the Year Edition.