If you want 'free' (as in no monthly fees) one of the best ones is Guild Wars. You have to pay for the game itself, and the expansions if you want it, but you don't pay any monthly fees.

If you luck out and get a job, World of Warcraft is the most played game in general. Take a peek at this 'MMORPG' market share chart.
http://mmogchart.com/Chart7.html

WoW takes up 52% of the MMO market share. That's ONE game beating out ALL the other games combined. And it is probably one of the better MMOs out there.
FFXI may be a little frustrating for you, especially if you're coming from Maple Story. WOW is really top-of-the-line.