I think he had a prime, he had already reached his physical and skills peak by thirtheen and once he activated his ms, the point where he mastered the three techs efficiently and found the weapons would be his prime. then his strength was on a slow downward slope as his vision became more blurry and he couldnt just pop out his ms for everything like sasuke did.
but yeah, that's why itachi isn't in my top ten. it's shown very clearly that against multiple opponents/bunshin his style isn't nearly as effective, whereas there are at least ten characters who've taken on multiple kage levels and won and a few kage who we haven't seen nearly as much of as itachi and who would probably be in a similar level.
category one: hashirama, madara, obito, dsm kabuto, naruto, sasuke, hiruzen, minato, hanzou, nagato
category two: muu, second mizukage, third raikage, oonoki,kakashi, itachi, sannin, gaara, sasori, white fang etc.