Myst Online, even though I've never actually played it.
Bacn is a sucker for the Myst series, even if Uru was horrible. Just can't beat the feeling of the D'ni city. Unless of course you could care less about Myst.
The online one was great from the demo I played..
It's a shame though I never really DID get to play with some people,
And It was the first myst I played that was free roam in first person view AND third person view.