This is a tough one.
I love WASP but wouldn't call Blackie a great singer. He's just perfect for WASP though.
Belladonna is a great singer but most of his Anthrax stuff isn't suited to his voice and really the only album of theirs I like is Spreading The Disease where Joey's voice is fine. Just listen to him singing Journey songs live on Youtube- he's got a great melodic rock voice, just no-one's heard it.
Going to choose Blackie as he suits the material he sings. If I was putting a band together I'd choose Joey though.
And I agree with Wolfie, TCI is a great album.