I really like everything, with an exception here and there.
But my favorite is ceartainly RPGs.

The adventure aspect is by far the most vital part of a memorable video game experience. Great stories can be found in books and movies, but only a video game is able to deliver an experience where the player is able to immerse himself and take control of how exactly a story plays out.

Notable games include The Legend of Zelda: Ocarina of Time, The Elder Scrolls V: Skyrim, Halo: Combat Evolved, Star Wars: Knights of the Old Republic and the Pokémon series.

Action/Adventure, with Zelda on one end, Uncharted at the other, but basically everything here is good.

Then, Construction & Management which I guess is a subgenre of simulation, with a SimCity 4, Anno and Rollercoaster Tycoon. Sadly the last good game in this genre is at least a decade old (with the exception of maybe Cities: Skyline, need to play that), but I'm happy to replay everything until the end of time if need be.

Third, 3D Platformer, which is almost solely Mario, but it does it well.

After that there's RPG, 2D Platformer and Puzzle which are pretty mixed, some games like KotOR, Yoshi's Island and Layton respectively are terrific while others are not. As a rule of the thumb I'd like to keep it simple, I always say "if there's too much numbers on the screen, I won't bother". Then, there's genres I don't really like but there's still a good incidental game or series, like Racing with Mario Kart, FPS with Call of Duty (the first two) or Fighting with SoulCalibur.

Its easy. RPG's.

With that said this so called action/adventure category always pisses me off. Especially when shooters fall on them like they are anything more than 3rd person shooters.
Its juch an unerving category. Every game is an adventure and 90% are action games. They really need to sort it. The "whatever doesnt fit on our popular categories" really has to go. They simply need to better define what an action game is to something where you fight without guns for the most part and drop the whole adventure nonsense.

RPG's, Platformers and action adventure games!

I also like Survival Horror games but I have an issue with this genre... for example, I love Resident Evil games but I can't play Silent Hill. You could say I like the genre as long as it's more about survival than horror.

RPGs and Action/Adventure

RTS. Followed by Action/Adventure

Deus Ex original.

Platformers ( both 2D and 3D)! A platformer will make me very happy most of time, regardless if it is Super Mario World, Conker or Rayman Legends!

I would say, adventure RPG, or just action adventure