Black Dragons are equally as bad. Terrible damage output and they can give themselves total concealment, makes for a long and very boring combat.
black dragons, wraiths
I can completely see the logic of this - yet, the only dragon encounter I've used so far was with a black dragon, and it played out really well. And I've used wraiths two or three times - always mixed with other monsters - and I think have kept things on the side of "will we make it though this?" rather than "what a boring grind!"Another vote for wraiths. I mean, thematically, all of their abilities make sense and sound cool on paper, but in actual play they are annoying and grindy.
With the wraiths, it's helped that the party has multiple sources of radiant damage, so often the challenge has been to find a way to bring that to bear in order to end the regen/aura.
With the dragon, it helped that the party was carrying a statue of the Queen of the Summer Courts, and decided to use it to try and negate the dragon's darkness aura (succeeded twice with escalating Arcana DCs, failed the third time round).
I did use these once, in a setup where I thought they might be able to get CA to help alleviate the -4 clumsy penalty. Didn't really work, though . . .I like the flying zombie that gets -5 to its attacks whenever it flies. Did I mention it has a special flying attack? What a well-planned monster!