Fenixp: X3. It's like another job, only more time consuming.
Titanium: This.
lotr-sam0711: Morrowind. I could play that game for hours and not do a single quest, just spend all my time exploring the world and checking out all kind of nooks and crannies I'd discover in the world.
Titanium: That one too.
In fact, all open end games (or games that you can finish with multiple classes or different endings) have a tendency of taking up an enormous amount of time, if you're not careful. Morrowind... yep, had a bit of a slip there. Put lots of hours into it but in the end well spent, I think.
Oh yea, Morrowind definitely,I've spent at least 400+ hours into it and I just started a new playthrough with the Merged Morrowind Mod Megapack which adds over 100 mods=). And the best part is that I never feel like the time I played Morrowind was wasted.