It's a really really hard choice for me, most fun I ever had in any MMO was the beta for Matrix online, I hated paying for that game it was terrible the people on were awesome though.
Asheron's Call being my first MMO had me the most Captivated.
My best PVP moments came from a split between Fury and Star Wars. Killing Jedi's during permadeath days were awesome and on Fury since it was pvp 24/7 you could shank people about to sell in a town and sell all their stuff, epic.
Other than that they've all been just there. Nothing really kept me around like Asheron's Call did, DDO doing the next best thing but after a 15 month tour in Iraq it made DDO seem all fresh again so it was like a second lease on the game.