Or console game, for that matter.
Mine would have to be the origional Sim City with Chip’s Challange in a close second.
Yours?
And if you know where you can download your favorite classic game include the link.
Populous
NetHack
Gotta go with Empire.
Civ 2, easily.
Moving this to Cafe Society.
Oh, and my favorite was Might And Magic.
Dark Castle!
But The Secret of Monkey Island is a very close runner-up.
Alpha Centauri
Interstate 76. One of the best car shoot’em up games I’ve ever played that included a sidekick with a huge afro, funky 70’s inspired music, and the ability to play human opponents on the net.
marc
I second Civ 2. I am currently working on a massive Viking Empire 
The Secret of Monkey Island! Best old-school PC adventure game ever, IMHO.
Cicuit’s Edge was also awesome. It remains the best cyberpunk themed game I’ve ever played. (Not that I’ve played that many, though Shadowrun for the Genesis was a lot of fun.)
Monkey Island 2: Monkey Island with better jokes, better graphics, and a spitting competition!
And Dark Castle for the mac.
XCom: UFO Defense - pure gaming goodness.
I’ve been playing computer games since Zork, and I hate to single out favorites. But when I was in college, I remember sitting down with a buddy to play Star Control 2. We only planned to play for an hour or so. 29 hours later (!) we finished the game. We barely even noticed. It was that addictive. And it was totally worth it. It wins the “can’t stop until you’re done” award.
For the “repeat play - unmodified” award, I’ve lost more hours to the original Civ than I care to admit.
For the “repeat play - modified” award, I’ve got to go with Thief and Thief II. Not only were the original missions great, there’s some damned nifty fan-created missions out there, some of which are better than the original missions, and many of which have the whole nine yards – briefing movies, in-game dialog, crisp storylines, the works. One clocks in at around 330 MB (!). They’re available here. RIP Looking Glass Studios.
And I’ve got to give due respect to the Ultima series. There’s a dandy little app called Exult that allows it to work under Windows, and even adds some interface enhancements (which can be switched off if you want to play it “as it was”).
Incidentally, there’s a great message board devoted to getting old (mostly DOS) games to work under new computer operating systems – VOGONS (Very Old Games On New Systems), if any of you want to fire up old classics.
I don’t know if it’s old enough to be considered a “classic” but Curse of Monkey Island is easily the best game of the series. A hell of a lot better than Escape From Monkey Island at any rate.
ALL YOUR BASE ARE BELONG TO US!
Either the original Zork series, or Bard’s Tale. I look on those games with a great deal of fondness.
Be careful here. The SDMB frowns on posts encouraging copyright infringement. I’d refrain from any direct links unless you’re sure the game is in the public domain.
If you’re looking for old games, Home of the Underdogs is a great place to start.
Though a lot of the games they offer for download are in the public domain, a lot of them aren’t, so if you wanna go there you’re gonna have to Google it.
Doom
Ultima Underground
Half Life
Deus Ex