For me it's all about a unique sound that appeals on a new level. I'm not much for being weird for the sake of it, but some guitarists just have that 'certain something'.
Denis D'amour fused the whole metal thing with delay pedals and space rock. The band were obsessed with science fiction and his guitar sound complemented that aesthetic perfectly. Lots of synthetic scales and an almost robotic, jagged feel. Perfect for metal. Very pleasant to my ears.
Jeff Waters is very imaginative. His ideas are colourful and performed with incredible technical skill. Instead of being yet another generic shred guitarist, he chucks interesting little features into the mix, using toggle switches, effects pedals and unconventional jazz styles. It never gets boring (even if the vocals are questionable). He's incredibly tight as well - every single note and beat is 100% accurate and the tone is perfect. It makes for a very tidy, clean sound.
As for Devin Townsend - he's the master of being 'full-on'. He has a sticker on one of his guitars that reads 'FUCK SUBTLETY' - it sums him up perfectly. If I want to hear total extremity in guitar, I usually reach for a Strapping Young Lad album. Some of it is really easy to play, but it feels angry as hell, which is a lot of fun. Scott Ian from Anthrax has a similar appeal.
I could talk about these guitarists all day.
Bill Steer is a good one. I like the way his guitar playing is constantly evolving. He sounds different on every release.
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