My top genre would probably be the RPG. I've noticed recently that I've been playing them more for the storylines than the actual gameplay. In fact, I tire of turn-based RPG gameplay easily. That aside, the thing that has me stuck to this genre like glue is one simple fact: I'm a sucker for any good story involving anime characters.
My second favorite genre would probably be platformers. Classics 2D versions like Mario and Rayman, and 3D titles like The Legend of Zelda, Majoras Mask in particular all fall into this wonderful world.
Racing comes in at a good 3rd. F-Zero, Mario Kart (both the SNES and GC version for the aforementioned two), Need for Speed, Grand Turismo... Ah, so many titles, so little space.
Fourth is scrolling shooters. I've enjoyed many hours of Galaga, Gradius, ZeroWing, and now Touhou.
I'll throw out a 4.5 spot while I'm at it. This goes for First Person Shooters. Half Life, Call of Duty and Portal (Technically not a FPS, but close enough) have made too much of an impact on me to leave unmentioned.
For 5th, Fighting games take the spot. I didn't get into fighting games until recently, but once I tried my hand, I was hooked. Spectacular gameplay giving you the thrill of the fight without the pain the next morning. One of the things I love about 2d fighters (Street Fighter, SNES DBZ) is that each frame of a character's attack was hand drawn, or in Clay Fighters case, crafted 'n captured. Today'd 3D fighters (Tekken, Dead or Alive, PS2 DBZ) still hold solid ground here, but lack the visual attraction that 2D fighters held for me. Even still, they are visually stunning in their own manner.
And as a bonus, I'll mention a genre often overlooked: the Visual Novel. Some people don't think about them often, but this Japanese genre boasting games like Phoenix Wright and Hotel Dusk have made a sturdy niche in our American market. Incidentally, I have a few roms in this genre that I use as aids in my study of Japanese language.
I'll go ahead and throw out the fact that I cannot abide sports games. If you're gonna do it, go outside. :|