Definitely Dragon Age: Origins. . . . PLAY IT! Not so much, but still recommended by me, is DAII.
Haven't played the other two, so no comment. I could regurgitate what I've read about them or what other people think and pass it off as my own opinion so I sound well informed, but that isn't very honest of me.
Dark Souls. You'll love to hate it. Remember those old games, usually RPGs or platformers? They seemed impossible, but you felt like a god when you beat it. That's more or less Dark Souls. A real challenge and a solid RPG.