Just to comment on a few mentioned so far-
Don Dokken - Agreed. He's really lost his voice and range. I've checked out a few live clips of him, and he just can't hit the notes. And I don't even mean the really high ones, I just mean the average mid range ones.
Vince Neil - He was never a great singer, although he seems to have kept his range. His main problem is that singing the full lyrics and correct phrasing is optional for him now. Just listen to the Carnival of Sins DVD, and it's painful. He'll come in late for every line, then only sing a few words out of each line. I think it's too much effort for him to sing an entire line. I'll be seeing them live soon, so hopefully it doesn't ruin it too much.
Ozzy Osbourne - He was never a strong singer, although he's actually done some much better vocals than he gets credit for. I saw him live a couple of years ago, and while his singing wasn't great, the guy was giving it his all and doing his best, and even hobbling around he managed to get the crowd into it. I admire him for doing the best with what he's got, even if it's not very good anymore. His last two albums feel like they've relied heavily on pitch correction to even get a melody out of him anymore though.
James Hetfield - I think he's actually made a good recovery since DM. He went through the long "Barney" period (I call him Barney because he sounded like Barney the Dinosaur), but recently he seems to be making an effort to growl it out like he used to without trying to overextend himself. I saw him live this year (or was it last year?), and he was surprisingly great. I really couldn't fault his performance. Maybe not what he was 20 years ago, but for the music they play, he's still really good.
Axl Rose - I haven't heard anything from him in the last couple of years, but when I was checking out recent live clips of him in 2008 or so, I was actually quite impressed at how good he was. For a long time he was dreadful because he just wasn't doing anything, but he seems to have made a good comeback.
David Coverdale - Coverdale is still great for his age. Live he's not too consistent due to his age, but on record his voice is still great, especially on the softer stuff where his tone has only gotten nicer.
I can't think of any of my own picks right now. I could name a tone of vocalists who have held up well though.