I'm only a casual dir en grey fan, but this is still my favorite track from them! Great choice, I absolutely adore that opening riff.
Another one of my fav. dir en grey songs. I love how "big" the riffs are in this one.
As for some of my personal favorites, here are 2 that came to mind. Neither are particularly creative or musically jaw-dropping, but they've struck a chord with me and have stick with me throughout the years.
凛として時雨 - replica (the riff starts at 1:23 and plays throughout most of the song from there. you can hear it particularly clearly at 1:45, though it's rhythm is slightly different at that point).
It would be easier to choose a riff from ling that ISN'T awesome. TK's vocals may be troublesome for some, but he's an absolute demon on the fretboard. More than his technical skill, his sense of melody and knack for coming up with distinct, yet catchy riffs has always awed me. I love this riff in particular because it's kind of wacky-sounding, yet furious and intense at the same time. This one also feels a bit more "thematic" than a good bit of TK's other songs since it's used a good way though the song and not just as an intro/outro thing.
9mm Parabellum Bullet - Termination (the main riff here - starting at 00:10. it plays a bit throughout the song. You can hear it for a bit longer @ 3:18 as it closes out the song, and eventually ends without distortion)
Yoshimitsu Taki was, at one point, one of my favorite guitarists next to TK. A lot of his riffs in early 9mm material were catchy and had a distinct style to them. I enjoy this riff because it sounds beautifully out of place. It sounds like it was lifted from a cheesy European folk metal song, yet here it is in a "hard" "indie-rock" track? A wonderful microcosm of 9mm's subtle genre grafting at the time.
EDIT: Here's another really simple, but effective one that i'm fond of:
OGRE YOU ASSHOLE - ロボトミー (opening riff, used throughout most of the song)
I love how OYA took this one, repetitive riff and expanded it into what is imo one of the best songs they've ever done. It's simple, but it has a groove to it that sucks you in makes you nod along; almost like that of a really sleek hip hop beat.