This one's easy for me: his late 70's "Back to the Egg" era vocals. It's perhaps my 2nd most played Wings album, and it's not because of the material. I've always felt his rock growl in this period was unmatched. And, it's a pretty rocking album to begin with, so it's a great showcase. His belting tone during the Bealtes (ie Helter Skelter) was also great, but I guess I just prefer that something extra that I hear during BTTE.
I've wondered if it had anything to do with smoking, as to why he had a particular gravelly "bite" in this period. I'm not sure, but listen to examples like "Getting Closer", "To You", "So Glad To See You Here", and "Old Siam Sir"! Not his best lyrical period, but who cares when you have a sound like THAT. Never did Paul rock harder, imo.
And in the middle of that, he could slow it down a bit with a vocal like "Arrow Through Me", which I feel is criminally underappreciated. What a great R&B soul tone. You get the feeling Paul was showing off a bit on this record; he must have been feeling particularly confident vocally...