World of Warcraft, Vanilla through Burning Crusade. Started back in 2005/2006.
My 1st proper MMORPG, I remember fondly my 1st couple of years, the people I've met, the friends I've made, the sense of discovering lands that I wouldn't dream about, the fun PvP (albeit inbalanced at points) and the fact that I went from a newcomer, noob, player to getting into a top raiding guild on my server.
Man, I spent thousands of hours only to quit halfway through cataclysm but every now and then I look back to the start of it, those were the times.
I'd give anything to get that feeling back but the game's just way too different now unfortunately (for me)
and even if I get the feeling to get back, I know I'll only end up getting disappointed.