well, i'm not entirely sure which i would say is my favorite but i do occasionally return to the game, Nodiatis. one thing i've always like about that game is the owner, Glitchless, does hold to his word. you use a time card to get paid or premium account status for a certain number of days and the character you used it on will never be deleted unless you do it yourself. so...around 5-7 years now and while i get the periodic email reminding me of it, my original character STILL SURVIVES! those passive skills, the passive exp gain (passive training), is all STILL rising to this day without end.
RuneScape. Although I dislike the game in its current state, back prior to the game-changing update in December of 2012, it was easily my favorite MMO and go-to game in general.