If I had to pinpoint it, I'd say Flash's primetime was around 2005-2008 perhaps, and FlashDevelop was available then. Guess we probably define it's prime differently haha, I'm thinking more of when it matured - AS3 as a language, lots of tooling choices, etc.
I wasn't ever anything close to a professional Flash developer, I'll take your word for it if you say that was the best time to be developing for it.
I was thinking about the days of Homestar Runner, Weebl and Bob, Newgrounds, and so on, when flash cartoons and games were (for kids, at least) a huge part of internet culture, and everyone wanted to be a Flash animator. Youtube kinda killed the Flash cartoon medium, sadly. Sure, videos are simpler and don't rely on a proprietary binary blob, but there's nothing like loading up a Strong Bad email and clicking random things (or, uh, holding down tab) trying to find secrets.
Ah, don't give me too much credit haha, was more of a side-project thing for me, definitely wasn't a professional, especially at the animation side of things (as opposed to the programming side). I was also more involved with the games side of Flash, which Flash became much stronger at as ActionScript 3 came out which coincided with much better Flash performance. Flash advertising and simple animations were probably stronger earlier.
I'm just interested in the topic because it's kind of neat to look back at the internet and observe its history and the changes its gone through. Just did a little wikipediaing for fun - here are when a few different websites / notable games were released:
