Metal roof?I haven't used enough to have a best and it might not even be what you're looking for, but I used this on my house and it's held up the 5 years it claims.
Metal shed roofWhat are you using it for?
I know people in the camper world are saying good things about Henry's Tropi-cool. Really they seem to say good stuff about any of the Henry's products, but they like the Tropi-cool because of it's reflectivity helping with keeping things cooler.