it depends on what its like .. im into the questing .. and spell casting ect.. like wow.. but on ps3 XD
Though my most thoroughly played Open-Ended RPG is Oblivion, Morrowind is not far behind. Though it is fairly old, Morrowind is a jewel in the open-ended SP RPG world. If you end up getting it, make sure you get the Tribunal and Bloodmoon expansions, because there are more quests, more armor, more weapons, and more fun. I'm not sure if the expansions are on console or not, however, the PC version does not require the worlds best gaming computer by far, so consider getting it for PC.
Oblivion? Get the Shivering Isles expansion, and become Sheogorath
I am getting in to TES modding for the PC, it is fairly fun, and you get to make your own quests, dungeons, and fixes
Anyway, Raunch - Stop being a GH obsess, thats Daily's job.
My advice is to not play MMOs. The quests lack depth in most (including WoW, all it is is killing monsters really), so I would recommend that if you wanna stick with RPGs, stick with single player. Diablo 2 online is great, though. Even if there is only 5 Acts (including expansion) and less than 30 quests.