Dio was a giant among men when it came to his voice. Ive heard his personality was exceptional aswell. Glenn Hughes speaks highly of him.
Hughes is another one of my favorite vocalists, such a great set of pipes on him.
DT takes some time to get to know well and they're unusual if you're not familiar with Prog, but theyre worth the effort. They have many great albums. They do tend to wank a bit and can take a moment to get to the point, but their overall songs are exceptional. Many choose to focus on the former, but the latter is worth it.
You should check out Symphony X too, they're very proggy, with heavy heavy riffs, blistering solos but a bit of power metal and theatricality thrown in, very epic.
Check out Paradise Lost and V: Mythology Suite, those two are fantastic. Not a bad song on 'em.