Nothing necessarily criminal so far, but Hannity was a secret client of Cohen's that Hannity didn't want people to know about it. By standards of journalistic integrity, Hannity should have divulged that he was a client of Cohen's while reporting on Cohen to avoid appearance of bias or conflict of interest, but it's Hannity and Fox, sooooo.....
Hannity says he was never an "official" client of Cohen's, and Cohen just casually gave him some legal advice once upon a time at a party or something, but Hannity was referred to as a client of Cohen's in court.
Hannity has used shell companies to buy up a bunch of expensive real estate, but again nothing necessarily criminal there.
But Hannity is currently erasing any tweets connecting him to Cohen, tried to hide being a client of Cohen's, and downplays it to an extent that seems to conflict with court statements, so it makes people wonder if Hannity is hiding anything unsavory that Cohen might have helped him with, like paying off mistresses or anything shady with his real estate dealings.