1) PWI (The only reason why this is number 1 is because of its sentimental value to me)
2) Atlantica Online (Very unique gameplay. It's a turn-based MMO, with an element of a Tactical Battle System in it, a la Fire Emblem)
3) Final Fantasy XI (It's an old game, and once in a while I reactivate my account. Only reason why this isn't higher on the list is because of how the game evolved into a gear wank)
4) TERA (This game has potential, and I wish I had a chance to play it more, but as with many games, it's a lot more fun with friends)
5) I don't have a fifth. I love playing MMO's, and I've played many. But only 4 has kept my attention for more than a couple of months.