Your choices are a bit limited with a mac. It's unfortunate, but Windows is dominant when it comes to computer gaming.
Planescape: Torment (Mac) is one of the best games I've played, but is very focused on story and exploring ideas. It's combat is quite poor and archaic.
Dragon's Dogma (360) has one of the best combat systems in an RPG, but is very lite on story.
Dragon's Age (360) has a decent story (a bit like Game of Thrones in places) and also a solid combat system.
Skyrim (both) has lots to do, is fun and will eat up hours. It can get repetitive though, after a while every dungeon looks the same and the combat gets dull.
Fallout 3 and Fallout New Vegas (360) are a bit different from the usual fantasy fare in RPGs as they're set in a post apocalyptic wasteland. Think guns and grenades instead of swords and sorcery. New Vegas (IMO) has a better story and setting. Both have the same combat mechanics.
The Witcher Series (1st mac only, second both) is set in a traditional fantasy world, but uses it to explore social issues and conflicts. Combat system is solid and enjoyable, but not a lot of versatility compared to other RPGs
Your choice really depends on what you want in an RPG, but these are at least a starting point.