Those are the first ones that came to mind. I couldn't possibly keep count of how many times I've played them start to finish. Even if I wasn't in the mood to play either one if I started playing I could just keep going until the end.
Probably Warcraft 3, including the expansion and DotA 1. It's also the game I've played the most (except for maybe WoW, is probably close though). Mostly for the multiplayer, I don't know how well the campaign holds up, I haven't played it in quite a while. I remember it fondly, but it could be nostalgia goggles on my part.
Can't claim that I've played any game that's perfect, but this one. This one gets really, really close. At the very least, it's as perfect as this kind of game gets.
This game has some insane timeless qualities to it, and in a lot of ways embodies so much of everything I love games for. It's got an established universe with some clever rules and great characters, it has some of the best control in any game I've played and it feels so good. The aesthetics are a complete wonder and the combat is amazing with so many ways to approach an enemy and look cool, and there's some devilishly hard challenges that feels rewarding to master.
The story is something I feel everyone can be able to appreciate. When I was young it did a great way in making me feel awesome and I loved feeling like a hero. Playing it now still brings back some of that childlike wonder. The music is also absolutely awesome.
And at the end of the day when you've finally thwarted the bad guy and your heroics are celebrated, the icing on the cake is getting one of the coolest credits sequences with one of the best music reprises. Next to Mario Kart 64 I don't think I've ever played a game that's made me feel that good about finishing it. It also says a lot about the developers, having put so much enthusiasm into making something this awesome. It's something I sorely miss in games. Brings a kind of nostalgic tear to my eye.
Bungie's Halo games are, by far, my favorite video games. At this point, my total time played in Halo games has probably surpassed the sum total of time I spent playing other video games, which is surprising, but I'm still happy I got into them and can't wait for Tiger to be officially unveiled.
Can't choose between Mario Galaxy 2, Deus Ex, Metroid Prime, and Final Fantasy VII.
SMG2's and Deus Ex's gameplay mechanics are perfect (though in opposite ways), Metroid Prime and FFVII have the best game worlds I've ever experienced (and probably my favorite soundtracks in a video game) to go along with terrific level design from Prime and my favorite characters from VII. All four were unreal experiences.