ALL of the above, plus the whole visual aspect of a nicely-set-up drum set. Ahhhhhh. Sometimes I go into my drum room and just stare at my kit for the pure visual pleasure. I remember seeing Ringo on the Ed Sullivan Show (yes, I'm that old!) and being fascinated by the way his cymbals rocked back and forth on their stands after he hit them. Little things like that are a visual treat.
Hi-hats! So many possibilities from absolutely closed to wide open, barks, different surfaces, foot pressure, flourishes, and combinations. Close second for me would be the bass drum, because I love using my right foot not just for the pulse, but for shadings and flavor too.
My 24" Sabian APX Ride cymbal which I use all of the time. It's the most versatile cymbal I've ever had.
Closely followed by my Gretsch Catalina Mod kit which always draws compliments. It's not an expensive kit, but it's designed in a manner which makes it look more expensive than it is. And the bass drum is monstrous in a good way.