Best PC game of all time?

Since the debut of the very first personal computer game in 1951, there have been many versions of computers, operating systems and (I truly have no idea how many) games have been released for each of those systems. Well, obviously, there are simply so many games that it's impossible to have played through and completed all of them in one lifetime. So out of the ones that you have played, if you had to decide on just one, then what would you say is the single best personal computer game in the world (I'm mostly referring to the games from the period of time between DOS and partially DOS operated systems to the games of today)?

I can already tell that many of the more "experienced" gamers here will go for Doom, Quake, Unreal etc. because those were really quite revolutionary back in the day, but I'm still interested to see some other opinions for newer releases, contributed by the younger members of our community here at the GFD forums. I personally cannot decide on just one computer game and call it the best of them all, but I know there are many who can and I'd be interested to see where this goes. Also, if you've never played any of the games mentioned here, feel free to use this thread as a recommendation topic (I know that I certainly will!).

i have two:

The lord of the rings: The battle for middle-earth - honestly the best RTS i've ever played and i still wish i could play it or even have time for it, it was one of the first games i had played on the computer

The other is WoW - This is just an amazing game which has many fans, but i'd put Battle for middle-earth first