Mastodon - now, I do like many of their songs, and I love their themes. But listening to an album is hard work - it's just noise. The drummer overplays far too often, the riffing gets too sludgy a lot of the time, and many songs have no breathing space and if you're not concentrating (i.e. listening on commute), you lose track of what you're listening to. I don't dislike them - the dual singists thing is good, the soloing is often very tasteful, I love Troy's bass noises.
I prefer Megadeth to Metallica in terms of number of songs I'll listen to. Little Dave wrote some brilliant basslines that you can actually hear. Yes, Dave M is a gigantic plonker, but he's got some good tunes.
My favourite Metallica albums are Load and Re/Load.
I have a blank spot for acts that sound like they're between Rawk and Metal - Hard Rock? Heavyish Rock? Metallish? *takes deep breath* Thunder, The Almighty, Black Stone Cherry, etc.
The Tragically Hip ought to have been successful everywhere.
I wish Jordan Rudess hadn't joined Dream Theater.