I believe Itachi is very strong, it is stated that anyone who takes on a uchiha 1 on 1 is at a disadvantage due to there sharingan. I think anyone who has knowledge of uchiha fighting skills and genjutsu would be able to try and fight but the fact that all you had to do was look at itachis finger made it pretty frickin hard!
Hashirama vs Itachi: Hashirama because he defeated Madara
2nd Hokage vs Itachi: I think Itachi would win only because we really don't know much about the 2nd hokage and he didn't seem to have the mokuton ability like his older brother which seemed to negate the sharingan's abilities somehow.
Jiraiya vs Itachi: this one seems to be difficult because itachi even said the best he would do is a stalemate. but i don't know if that is because he actually believed that or he had other plans for naruto and the nine tails.
4th Hokage vs Itachi: We saw the 4th take on madara but madara really didn't use any genjutsu, and if itachi used susanoo i believe that he would just yellow flash and dodge the sword until itachi ran out of chakra. but if itachi did use sharingan to put him in genjutsu, 4th would lose cuz 4th could easily dodge amaterasu and susanno
And what prevents him from using genjutsu on Hashirama and Jiraiya? Neither of whom have a defense against his genjutsu. Or any of his MS abilities