Surprisingly, perhaps, it's WoW – we used to run around in a group of three and conquer the entire server(!), but nothing worked... But what fun it was! :) And now there are many players, and everything works – but the fun is gone :(
Also, probably Pharaoh, an old game that's similar to Caesar 3. But again, you can't recapture that feeling...
And Age of Empires 2...
And a little bit of Heroes of Might and Magic III :)

But most of all, I miss the old forum; yes, I'm getting old, but back then we communicated, and didn't just engage in pointless chatter. That's not the same thing :-/