Runescape. A game very deep,
with a lore you could drown in, different gameplay styles, the main character having complete reign of their development, and hundreds of different tactics you could employ.
If it were single player, there would be much more room for a more expansive lore, fleshing out of areas, ect. For example, the Kharidian Desert is an area open to you from very early on; it's also very high levelled, and all of its quests are painstakingly tedious. However, if you go out there, nobody's doing quests there. Same with the Wilderness, where are PVP is on and all enemies are hostile, even if you could kill them by staring at them.
And its fanbase are a bunch of stupid, sometimes even racist, stubborn wastes of human life (It is a British game).
Runescape would also be much more fun if it had a party system, as opposed to making conga lines that can get very easily seperated.