Well, I'm no vocal coach, but regardless...
Roy's just got a wonderfully unique voice - his enunciation is wonderful and although his range isn't what it used to be, he can easily find a way around it - listening to Poetry for the Poisoned, you can hear just how gifted he is, working his way through those really gentle high notes. He's got excellent pitch recognition. I guess what I love most about it is just how, despite not having the widest "power metal singer" range, he can still do so many things so well that a lot of other singers pale at in comparison.
Tobi just has an amazing vocal range, he's been doing it since he was a teenager and over a dozen albums later the guy's still able to hit those crazy high notes so well. His soft singing isn't as good as Khan's, in my opinion, but the two guys are pretty different singers to begin with. It's hard to pinpoint what exactly I like about him so much - at first I thought he sounded like Bruce Dickinson (like most others) but I honestly think he's much better than him, even in Bruce's prime.
Jorn... come on, listen to that voice. I'd kill to have that bluesy, hard rock scream ability that he has. And yet he can sing gently too! The guy's a monster on the microphone. Great range and he just has that sound in his voice that everyone seems to love - the end of Avantasia's Scarecrow song still gives me chills to listen to, as does his work on the longer songs of Ayreon's 01011001. That rasp in his voice is always something that has mystified me, I've never figured out how singers are able to do that, let alone do it without destroying their throats. Like Chity's work on Firewind
