Didn't think of Perry or Orbison, but those are also good choices.
Bowie I considered, along with a few others like Rod Stewart, Tom Petty, Jeff Lynne of ELO, Jagger, Daltrey, Clapton and even Elvis and Little Richard.
There are few guys in the metal genre I like as well, including Ozzy and Paul Stanley.
Alternative is quite a bit harder. Because you've got legends like Gabriel, Morrissey, Iggy, Bono, and Peter Murphy to contend with. And then a bunch of other good lead performers from the 90's and 00's like Cornell, Corgan, Robert Smith of The Cure, Grohl, Cobain, Thom Yorke of Radiohead, Scott Weiland of STP, Anthony Kiedis of RHCP, etc.... I also like alot of Perry Farrell's vocals for Jane's Addiction.
Of the older alternative types, I'd probably have go with Bowie, Iggy, and Murphy, I guess. More recent (as in 1990's-2000's), maybe Farrell, Anthony of RHCP, and Smith. I'm not sure they are
the best. But they're the ones I tend to enjoy listening to. I like alot of the others as well, including Weiland's vocals on the STP tracks, and some of Corgan's grittier/whinier Pumpkin stuff. Very hard to choose among these guys though! Which is probably why I chose the easy ones for my top three.