For me it would be Neverwinter Nights and Planescape: Torment. It was the first D&D game I played and being one who spent many hours playing D&D this game will always have a special place. I don't think I need to explain why I love the second one so much. Those who have played it will know why and those who don't need to play it.
Games that I end up playing nearly once a year, for varying amounts of time:
World of Warcraft (I actually do end up quitting for a while and returning once or twice a year)
And there are a handful of MMOs that I go back to not necessarily annually, but every so often.
I have lots. I tend to replay my games a lot, and since I've been gaming since the 80s (PC or otherwise) I tend to replay a lot of games.
To be me more in line with your question though:
Monkey Island (yes, all of them- even the bad one and the new one by tell tale)
Half Life 2
Splinter Cell (all except the new one which isn't splinter cell and I pretend it doesn't exist)
The Longest Journey
There's probably more, but these are what I think of off the top of my head. I try to play games I like again every year. This year had some new ones which it's been awhile since I could say that so that's pretty neat.
The only game that I have played every year since it came out is Civilization IV. I also tend to fall back to a Total War game (Rome, Barbarian Invasion or Medieval, haven't played Empire or Napoleon yet, though I will soon) at least once a year. There's quite a few other games I wouldn't mind making a yearly fixture, but it's hard enough as it is to keep up with new releases and a huge backlog.
Half-Life 2, Chrono Trigger, one of the KOTORs, Maniac Mansion: Day of the Tentacle, and most of the Geneforge series.
After playing Planescape: Torment for the first time this summer, I think I'll have to add it to the list. Red Dead Redemption and Alan Wake are also contenders.
I feel the need to bust out the N64 and play Ocarina once a year. Usually around spring break. Granted, I play it in between from time to time as well...but I need to do at least a full speed run once a year.
A friend of mine and his brother get together every year to do a hardcore run on Diablo II. That's a solid tradition, but man - hardcore mode is just so....stressful. I'm content with Zelda.