Dio was great, but there's a great amount of bias surrounding him. While he was consistently great during his golden era, he really only had one style he stuck with throughout his entire career. Plus since the mid 80's till his death he only really sang on one great album (The Devil You Know). Simone was never better though, but she gets my pity vote.
I even surprised myself here, but Phil gets my vote. There's no denying that Gillan in his prime had a much wider range, but Phil has a kind of a charm that Gillan lacked in his prime and that only really showed when he lost his range in his twilight years.