Detroit is at the top of my list for sure. Nothing can even contend with it's epic fail-ness.
Other cities I'd add in would include:
Winnipeg. I don't know why, but it just seems so boring. It wouldn't be
bad, but I think it would just be boring. This
is after being there for only about 4 days in my entire life, but it's the idea I get from it.
I'd definitely agree with Miami and Pheonix. I guess that they're both nice and warm, but they just seems like they've got a terrible demographic of people. Combine that with total car culture and they become a place that I'd only want to go for a vacation. And just barely.
Houston and LA are kind of iffy for me. Houston's got a fair bit of culture, and LA has some nice parts. But they've both got their downsides too. If LA didn't have the density of scale that 14 million people gives, transportation would've been a bullet between the eyes.
Interesting, I can't actually think of any "bad" cities outside the US (and Winnipeg, but I think my opinion is a bit coloured on that.)
Could this thread also include awesome cities? If not I'll be making another thread on that.
@BobBob: I lol'd