It must be. I was gonna ask "BEST for what ?" Obviously the best middleweights (Monzon, Greb, Hagler etc.) have the best builds for boxing at middleweight.
You're wrong...the gayest thread is currently on page 1 of the lounge.."Who the Sexiest, Sergio or Oscar"?:-(
I wouldn't say that any physique in particular is superior to the other. Plenty of short and strong powerhouses like Dick Tiger have been great middleweights, as have tall, lanky fighters like Hearns. In Muhammad Ali's view the ideal boxer looked like him and Ray Robinson, tall and in great physical condition yet not overly muscled. Perhaps there's some truth to it but as Joe Frazier showed, you do not have to look like a great "athlete" in order to be a great fighter.
It is Hagler. He was a middleweight from start to finish. 1973 -1988. He was perfect build for a MW and always in tip top shape. He could probably still make 160 right now!