i m sure itachi will win for several reasons :
1.pain s weakness is genjutsu and itachi s tsukuyomi is the strongest genjutsu
2.itachi knows pain s secret
4.even konohamaru defeated one of the them
5.when itachi was alive nobody attacked konoha (even toby says after itachis death that konoha isnt untouchable ,this is the reason that itaci is the strongest from akatsuki)
6.pain defeated kakashi harder than itachi
7.amaterasu will burn pain (itachi says that once you set the target enemy cant escape)
8.pains eyes are conected to each other and also to the truth (nagato) by using his genjutsu itachi could find out where is hidding nagato and will defeat directly
(if you have any doubt about any of these reasons tell me and i will explain more detailed)
1.pain s weakness is genjutsu and itachi s tsukuyomi is the strongest genjutsu
2.itachi knows pain s secret
4.even konohamaru defeated one of the them
5.when itachi was alive nobody attacked konoha (even toby says after itachis death that konoha isnt untouchable ,this is the reason that itaci is the strongest from akatsuki)
6.pain defeated kakashi harder than itachi
7.amaterasu will burn pain (itachi says that once you set the target enemy cant escape)
8.pains eyes are conected to each other and also to the truth (nagato) by using his genjutsu itachi could find out where is hidding nagato and will defeat directly
(if you have any doubt about any of these reasons tell me and i will explain more detailed)