AppleTV — because I want to extend my Mac’s desktop to my TV
Chromecast — because it’s so cheap I didn’t mind giving it a shot
AppleTV is more flexible, more powerful because you can run many third party apps on it, including interactive games.
Chromecast is a one trick pony — you can cast media onto your TV through Chromecast and that’s it.
I once considered Firestick (also cheap) but by that time I already have AppleTV and Chromecast and I figure I would not have time to play with yet another gadget. I can stream Prime Video just fine using the Prime Video app on the AppleTV.
Roku — I figure there’s no point having it unless I also subscribe to the Roku service, so it was never a serious contender for me.