EVE. Since there is no exp or levels, you don't have to worry about that. You could play the game for years and never get all the skills, so the real learning curve is coming into it knowing what you want to do. For fun, I would suggest combat above anything else, and it's fairly simple to map out stats in character creation and base skills.
Skill learning takes real time, and there is no impact on that time from you. In fact, you don't even have to be logged in to learn skills. I'm learning a skill that will raise my intelligence another point right now. This skill takes 6 days to learn, which is based on my stats. Since learning skills, such as stat skills, are based primarily on intelligence, learning this skill will reduce the time needed for all the rest of my learning skills. It's a very, very complex system, but it is not overwhelming.
EVE has the largest game world in existence, and the very best PVP. MMORPG.com gave them 4 awards last year: Best game, Best graphics, Best PVP, and even Best Company. That's how dang good this game is even after 3 years. And yes, it does still look very good. It got best PC graphics the year it came out from Gamespy, I don't know of any other MMO that has done that.
If you have any questions about it, let me know.