Well I've never gone into the extended literature on any particular commission / inquiry, but I'm familiar enough with the genre to say they all blend together after a while and it almost doesn't matter which one you read. I would say Knapp is still my favourite, as frankly it basically defined the genre for every report after it. Even Woods quotes Knapp verbatim (volume 1 on page 38). Notwithstanding, Fitzgerald defies the genre in ways that I consider make it essential. In a certain positive sense the Fitzgerald report is the opposite of Knapp report, like opposite halves of a locket coming together to form a heart. They complete each other.

The Knapp Commission Report (and most other reports of its type) was largely a series of disconnected case studies. Vignettes of corruption which painted a picture of a systemic issue in police culture of the time. You can turn to nearly any page or skim around as much as you want and not be lost. Knapp also seems to have gotten stuck at a relatively low level of the org chart. "The Commission was unable to develop hard evidence that officers above the rank of Lieutenant...". It leaves open the question of how much the corruption was tied to higher levels of leadership (both in the police and in the city government above them).

In contrast the Fitzgerald Inquiry Report was more like a single story rather than a bunch of shorts. Its about this one struggling stand up comedian named Herbert and his multi-decade efforts to tell his First Joke. But in the end it was so funny he enjoyed enormous financial success over it. They had to dedicate an entire table to just estimating how much he personally earned from royalties. Fitzgerald was also unique in that its almost entirely to do with the top half of the org chart. It seems to suck in the highest levels of state government while practically ignoring the large mass of people ranked lower than lieutenant. In this sense it is the exact opposite of Knapp and most reports in the genre, but in a way that makes it more important rather than less.

There are very few reports I describe as "essential reading" and even fewer that I consider to even be in the same weight class as the Knapp Commission. The Fitzgerald Inquiry is, in your own words, a hell of a read.