I think Amazon has Netflix beat on UX. To search Netflix without using third party tools is a real nightmare. The user interface seems user-hostile, like it's designed to conceal from the user the true breadth of Netflix's library (I earnestly believe it was.) It wasn't always this bad, in the early days of netflix's streaming business the search wasn't bad, and back when they were a DVD service (which is when I first subscribed) the search functionality was pretty decent even.
But with Amazon video, you've got the search features built into the amazon prime apps, which admittedly usually sucks, but you've also got the option of searching on amazon.com itself, or on IMDB (which has been owned by Amazon since the late 90s.) Searching on IMDB is what I do. When doing advanced searches you can specify that you're only interested in movies that are free on amazon prime in your region.
E.g.: Feature Action movies that are free on Amazon Prime in the US, and are in the IMDB Top 1000: https://www.imdb.com/search/title/?title_type=feature&genres...
(Perhaps IMDB advanced search for prime videos is bad UX for casual users (although it was my non-technical mother who brought my attention to it, so I'd push back against that notion) but at the very least it's an option. While I've seen third party websites offering a good proper advanced search of Netflix's catalogue, as far as I know if you stick to Netflix operated search interfaces you're basically stuck up shit creek.)