I would be in the minority here and say "Strange Highways". I've always love the dark atmosphere in this song. A perfect vocal performance from the man.
The "On Stage" album from Rainbow is chock full of great Dio performances, when he was more melodic and not so much growlie that he likes to do now. That's my favorite RJD era....when he was more of a singer. The only thing that could've made that album better would've been including Stargazer.
But, I'd have to say Kill The King (Rainbow), Die Young (Sabbath) and Don't Talk To Strangers (Dio).