Since these are some of my personal favorites, let me briefly explain what I look for in a good riff - the three most important factors for me are groove, hook and weight. I will take catchiness, steady rolling groove and simplicity over wanky, overdone riffs anyday and I think the best riffs are usually the simplest. Also, I seriously considered putting the entirety of "Dopesmoker" on this list but decided it was way too long. But anyways, take of this what you will - I ordered the songs alphabetically to make it easier to go through.
Hopefully you will enjoy this compilation and here's to another 50,000!