The only game I was NEVER able to put down was Warcraft III: Frozen Throne (seriously - it was so distracting that I eventually had to snap the CD in half to get myself out of the hole). Warcraft had excellant replay value because it was so easy to mod - the World Editor that came with the game was somewhat unintuitive, but was very powerful if you knew how to use it properly (the same could be said of most in-game mod applications, perhaps, but still). Almost all games eventually lose the thing that made them fun and unique (except Pakman and Pong, of course) - sure, you might come back to it in a month, or a year, or a decade, or your children might one day find it, but no game can keep you entertained indefinately.
By the way, is anyone who stopped playing WoW thinking of going back for The Burning Crusade?