Best Canadian music? Big and influential electro-industrial scene. Front Line Assembly (+side projects), Skinny Puppy, Will, Decree, Digital Puddle and practically unknown Phÿcus. Along with Norwegian Industrial scene, the most interesting in genre.
There are many great Canadian artists: Metric, Emily Haines & The Soft Skeleton, The Birthday Massacre, Crystal Castles, Holy Fuck, Tegan and Sara, Arcade Fire, Stars, Broken Social Scene, Grimes, Lights, Conjure One, Front Line Assembly, Delerium, Dragonette, Caribou, Leonard Cohen, Rush, Neil Young, Alanis Morissette, Feist, Parallels, Sarah McLachlan, k-os, etc.
Holy shit, canada is musical cancer. Avril Lavigne, Nickleback, Bryan Adams, Billy Talent, Celine Dion? Even Bieber, shite, I didn't know he was Canadian. I don't know about underground, but as for mainstream worldwide-spread artists Canada sucks a major one.