Anyone familiar with Grand Theft Auto: Vice City knows that it's a game with many modes of play (that's probably not the technical term): it's a driving simulator; it's a role-playing strategy game; it's an action shoot-em-up.
What it really is, I think, is a combination of all of these game structures. But the most interesting thing about VC is that, in creating its own alternate realistic world, it actually works on all of these multiple levels. Most games give you an escape from reality and are described as such--someone's had a bad day at work, so when he (and yes, I'll just be sexist and assume that in most cases it's "he") comes home he escapes into the heightened reality of his Playstation 2 console, blah blah blah.
The unique thing about Vice City is that it gives you an escape from the reality that it creates. If (when) I fail a mission, I can take out my frustration by driving around in a Ferrari listening to "More Than This," by entering in a rampage against gang members, by setting myself up in the parking garage with a sniper rifle, etc. Its multi-level reality, in other words, provides its own source of both frustration and relief.
Which isn't a revolutionary observation, I grant you, and I'm no gaming expert, so there are probably other games that qualify for the title of greatest ever; but I think it's why it's the only game that I could actually waste a weekend playing.