Prime is mostly rubbish (unless blatanly obvious a fighter is way past his best and shouldn't be fighting) because physical and mental prime occur at different times and, as Jaz touched on, it's mainly used to excuse a great fighters defeat.

He was too old, too green, had too much money, wasn't motivated because of his small purse, was weight drained, was too small moving up, had a bad training camp, needed to do a shit, had the wrong socks etc....

The problem with assessing boxing compared with other sports is most fighters only ever meet once, so whenever a close fight happens it's impossible to determine who the greater fighter really is.

Even matched fighters would probably need to meet a minimum of five times to get an accurate result. As many rematch results show, the form can be instantly turned round.