Favourite games come and go, allthough lately not much has been able to get me to play it through except Mass Effect 1&2, allthough I have to say that Mass Effect 2 already has a lot of "been there, done that" moment. If things go on like this, I'd have the third one as a novel rather than a game...
There are some I played excessivly as a kid, but that I think rather boring now, mostly Wing Commander Privateer, and allmost all space shooters except for the original X-Wing (at least, that thing is still a REAL challenge!). These are games I usually think "they're not that great after all, but I had tons of fun with them!
But there's three all-time favourites, that nothing was ever able to push from throne of awe, which I can replay even decades after and say, without nostalgia "this simply is a great game, and will always be!" That's X-Com: Ufo Defense, Star Controll 2, and "Das Schwarze Auge 2" (yes, a german game. Never made it out of our language, which is a pitty. It rocks every single D&D game, and it easily smites the new Drakensang to the ground, plot-wise, and most of all mechanics-wise). These games are that good and that much fun that I stil play them regulary despite dated graphics (allthough I have to admit that the full speech of the Star Control II port helps a lot).
I even have an Original CD of DSA 2 lying around, allthough you can by now download it as abandon ware. It's the only original I kept from my childhood days.