In my experience, the more docile breed roos are the best. We had a Buff orp roo, and he was gentle with the ladies, took wonderful care of them, and when a border collie was after one of our RIR's, he was chasing it. He also charged our great pyr - through a fence, but still! And quite a few other people I've talked to with orp roos say the same; calm, gentle, and very protective. I bet something like a brahma would be a good choice, as well.
If you go look at the reviews for buckeye chickens on here, there's a story of one roo who lured a couple dogs away from his hens. If all buckeyes were like that roo, I would have one.
The most protective breed I've ever seen
in person was a game cock. I was picking up a silkie and he was like, "come at me, bro," so if you want the best protection get a game roo. The downside is that he was very rough with the hens.
I can see a blue andalusian being protective. From what I've seen, the more "flighty" breeds tend to be pretty rough with the hens, though, but I bet that, given the higher energy, he would be a great flock guardian.
Hope this helps!, And
