My favourite virtual world is, if that counts, Tamriel, from the TES series. Especially Vvardenfell, because it is a volcanic island with unique fauna and flora, only surpassed in bizarreness by the Shivering Isles.
My favorite world is minecraft too. Also very good world in Half-life (all parts). And I recomend you all to play Metro 2033, The city is Moscow and it in postapocalypsice style.
I'd say Everybody Edits. It's a pretty good game, but the only problem with it is that you have to spend money to chat which is weird since it's only 1 cent.
Middle Earth from Lotr, Pandora from JC's Avatar, and then the Star Wars Universe. All 3 of those fully captured my imagination for prolonged periods of time in the past, which is hwy I am picking them.