I like some of my own riffs ;-)
[Congrats to myself on my ten hundredth post, haha.]
If you wanna check out an example of some own stuff I like... Look here, press "play demo" there (and it's merely a demo I came up with to do some sound samples for that custom guitar of mine). That riff is later played with an octave harmony/2 guitars. I like guitar parts with harmonic structure, be it riffs or solo parts.
It's hard to choose a Top 3. It also depends on whether it's sheer power, some cool melodic lines or a mixture of it.
Jeff Waters from Annihilator comes to mind as a riff master/beast with lots of crazy riffs. He can also be creative with open string riffs - I love the main riff in "Set The World On Fire" (actually I'm using this riff a lot when doing recording soundchecks). Here's that riff (starting from 0.33 into the track):
For something melodic... I'm thinking of Iron Maiden / that "Phantom Of The Opera" melodic riff. Maiden had a massive impact on my melodic style.
As for coolness... RAGE (guitarist: Victor Smolski / drums: (at that time) Mike Terrana) / that "Straight To Hell" intro riff - beginning from 0.11 in the link:
AC/DC have a lot... I always liked playing that "Riff Raff" riff.
Haven't been listening to metal for a long time... So many cool riffs escaping me.