Really hard to choose as it depends on my mood/level of intoxication but..
Forefather - Cween of the Mark @ 1:00 is great when drunk.
I'm not really a fan of Meshuggah at all but one of the first things that came to mind was the intro to Bleed. That shit sounds like pissed off mountains fighting each other or something.
The intro to Chaos Star by Unearthly Trance.
The Rotted - Kissing You With My Fists @ 1:45. I would never have expected something this melodic from them a few years ago.
Origin - The Aftermath @ well, the intro, the sweeping riff and the one prior to it all own.
The best riffs so far of the year are definitely on Spite Extreme Wing's Vltra, which is also by far my favourite album of the year. Couldn't pick one riff from this album without having to mention atleast 20 more.