i can pick three (in no order, as I'd probably place them as equals), but it's more due to their strengths as songwriters rather than plain showboating...
1. J. Robbins (Jawbox/Rollkicker Laydown/Burning Airlines/Channels)
Dude kinda plays somewhere between rhythm and lead, but always goes with strange kinda lines that really work well but always make you think "well, shit, i'd NEVER have thought to do THAT!" i guess i'll choose the song that got me into his work, "Outside the Aviary" by Burning Airlines. Nothing spectacular, I just like how he takes what could have been a straight forward poppy punk song and puts that post-hardcore twist on it, doing riffs that kinda start off as rhythmic chords and then go off on tangents. And you could pretty much sing along to the guitar lines in this song.
2. Devin Townsend (The Devin Townsend Band/Strapping Young Lad)
I'd say Devin is the definitive metal guitarist if you're looking for an all-rounder. Knuckle tight rhythms, brilliant solos (but not overdone), and in his solo stuff he's able to create massive walls of beautiful atmosphere with just a guitar. And as a producer, he's got a great mind for layering guitars for ultimate effect, whilst retaining the ability to pull it off live. For the best example of all his abilities, try "Wrong Side" by Strapping Young Lad.
3. Ginger (The Wildhearts/solo stuff)
When somebody first described Guns N Roses to me, they said they were a combination of 80s metal, proper metal, and punk rock with a bit of a glam twist. I felt VERY let down when I finally did hear them. However, The Wildhearts (who I discovered much later) were making the exact sound I had in my head upon hearing that description. Ginger has always been the main songwriter, and his mind for riffs and melodies, as well as slightly odd rhythms, is second to none. Nobody writes huge chunky-yet-melodic riffs like this guy, and his occasional forays into solos are pretty impressive too. His solo stuff is often a little less guitar-centric, but even the less in-yer-face guitar parts are still remarkably well written. For the best examples of his work, I'd go with "Sky Babies" (a 11+ minutes Wildhearts song about aliens (in a non-cheesy-sci-fi-nerd way) packed with giant riffs and guitar melodies), and his solo track "Black Windows" which somehow marries Police-style guitar parts with a strangely mid-peroid-Megadeth-esque instrumental section.