I would love to say Post Mob Rules Sabbath with Dio, but he followed that up with Holy Diver and Last In Line. I actually wish he and Vivian hadn't had a falling out. Would love to have gotten more out of that original Dio lineup.
If you think about it, it was almost magical how everything fell into place for Dio.
His band, Elf, opened for Deep Purple just as they were about to experience turmoil, so he hooked up with Blackmore to form Rainbow.
Then he left after three albums - a crying shame because Dio and Blackmore together where gold, actually more than gold, they were golden platinum and sheer brilliance, but he left just at the right time to join with Iommi and do the two Sabbath albums, and right after he quit, he was in a position to do Holy Diver.
So in a way I regret that all these endeavors didn't last, but... one project going on more would have ruled out the other.
Nothing on that list actually. I might have picked Iron Maiden but ultimately I think his leaving for a while was probably for the best, and Accident of Birth and Chemical Wedding might not have happened otherwise.
I agree with Maiden. A post Fear of the Dark album with Bruce would have not been great, and Bruce leaving let us have his solo carrer and then he returned anyway with the proper conditions to kickstart a reunion that lasts to this day.
And I forgot about Helloween. Yeah, another Hansen / Kiske / Weikath album in their glory days would have been awesome.