From Fat Joe's "My Conscience":
http://www.youtube.com/watch?v=zFXr78phmGA
Eargasm. It has that "organic/chemical" Alchemist style, it sounds dark and gritty as fuck, and it has a melody that will stay in your mind forever.
This was probably the beat that cemented The Alchemist as one of the GOAT producers. I'd put this against any other "best beats ever". Also gotta say that fucking Fat Joe thankfully killed it and didn't let the beat go to waste:
http://www.youtube.com/watch?v=Agc-LdNRtIM