The ability for a singer to produce a large variety of engaging musical sounds is really what I like hearing. I'd put the front vocalist for Yes, Beatles' Lennon & McCartney, both together and independently, Creedences' Fogerty strength and simplicity, Dylan's very involving and unique variety of vocal sounds, and Zep's Plant, minus the "sick bird syndrome" noise effects - screeching noises like a bird being strangled slowly to death. I felt it was over-used on too many live tracks, and to me, blurred or diluted Zep's great musical definition between their songs way too much.
Plant's new tracks with Cross sound very cool - much more relaxed and very musical.
There's so many - early Elton...huge musical variety and fabulous voicing. I saw him this year with Billy Joel and Elton was exceptionally good, even singing Joels' (they traded off back and forth) numbers solo.