Wow, now you're putting up a tough one. Either way anyone answers will have some metallurgists up in the trees.
This is purely personal and by no means absolute:
- Comparing Ozzy and Ronnie, the latter is probably the more refined singer in terms of vocal capabilities, technique and musical theory... but this is rock'n roll (or heavy for that matter), so the question is what really counts. I think there are few performances that exceed the rock'n roll attitude put forward but Ozzie in "Crazy Train" in terms of looks, lyrics, laughing, singing and all the wacky attitude.
- Rhoads vs. Iommi is not straight forward either. Here also the latter puts up a technically cleaner performance, while Rhoads sounds at times almost as if he would struggle a bit, although his shredding on this one is absolutely immaculate. Love the deep growl and his soli are also exemplary. For me Rhoads has the cleverer harmonics and the punchier riffs. Iommi does a little of the Slash-kind of boring phrasing at times.
- As an album Heaven and Hell is probably more refined and produced with more attention to detail, but for me Blizzard of Ozz rocks more and has more attitude. In Heven and Hell you already hear the long hair and lycra pants a bit entering the heavy scene later on; and of course Dio's penchant for mysticism and romanticism. In comparison Ozzie is just completely out of whack [emoji3].
- As you brought up Dio I just have to post this: I think this is the first ever music video made - and probably the worst ever. It's so shabby it is actually fun, but it also exemplifies Dio's vocal capabilities. For me this is Dio's "Crazy Train", cranking it up to "11".
Have fun:
https://m.youtube.com/watch?v=bkysjcs5vFU
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