for MMO - Ragnarok online. Mainly because it has so much re-playability. If only they would fix the bot issue
for PC game I would say Skyrim due to the skyrim nexus and all the great mods that change the game around to fit your taste
for console game - Dark Souls. Challenge in games is mostly gone these days. Its nice to have a game that is both hard and fun.
For classic games that are old, my fave would be Phantasy Star 4. To this day I would say it beats a lot of games :P