This is going to be the most shameless plug I've ever done in my life, but I actually believe my css selector engine ( https://github.com/chjj/zest ) beats every other one into the ground in terms of speed, features, and extensibility. I've probably just been waiting for an opportunity to shout it from the rooftops. Unfortunately, it's kind of hard for a selector engine to gain momentum since people don't really use them standalone unless they're writing a dom library.

It's not the coolest, but I think it's the coolest selector engine. I'm also totally biased.