I know it's been mentioned before, and I'll mention it again: Metalica.
Sure, they might have been good starting out, but good lord... their song structure is terrible, they don't even know the music they're playing, they do it all by ear, and every song seems like it's just one long solo after another, with absolutely no verse, chorus, or actually melody. Furthermore, the solos don't even follow how a solo should be: keeping in time with the melody of the song and augmenting it to show the art of the instrument!
Another band, I'd argue, is Disturbed.
Don't get me wrong, I love Disturbed and all their powerful glory, but they're not exactly music gods. Sure, they've got good lyrics, and a fairly good sense of how to structure a song, but it's not like they revolutionized music. Furthermore, their new album Indestructible sounds like they just kinda gave up on trying, opting for a "Yeah, we know we're awesome," kind of feel rather than actually working to sound great... bleh. I miss the days of Believe
And this is purely bias, I'm sure, but good lord. All of the (c)rap and hip-hop artists just are terrible (in my humble opinion).
Sure, you can say words to a beat, you can keep a decent bassline and beat. But of all the music I've heard, it's like "OH EM GEE, I AR GANGSTR!!!1!" and ends up sounding like they didn't even proofread their lyrics for grammatical errors. Call me picky, but anybody who uses "-a" instead of "-er" and isn't speaking Japanese needs to go grab a dictionary and learn to spell.
And the final band I'm going to nominate is none other than Bass Hunter. He's good, but he's not that wonderful, and after the ten-thousandth time of hearing "DotA", I'm about ready to rip someone's head off. Trance is good, and by no means do I want to sound like it's something that should be outlawed from music (like other genres...), but, at least in my circle of friends, there is much to be said about moderation.