I tend to keep a wishlist of things I'm interested in that are on KU, so that when I get a subscription I'm ready to go.
Maybe the best place to look for non-indie authors is books published by Open Road. They seem to cycle a selection through the service, and I've seen books by Julian May, Dave Duncan, Robin McKinley, Carol Berg, Kate Elliot, Patricia C Wrede, etc. If you do an Advanced Search for publisher "Open Road Media Sci-Fi & Fantasy" and then filter by Kindle Unlimited, you should see some decent stuff.
Other than that, there are a few books published by Amazon's own imprints. I think Marko Kloos is probably my favourite. Military SF.
And there are whole subgenres that largely live on KU, like LitRPG, if that's your cup of tea. Dungeon Crawler Carl by Matt Dinniman is a bit of a breakout hit for the genre, since it got a mainstream hardback release, but the ebooks are still self-published and on KU, I believe. Other big series are He Who Fights With Monsters by Shirtaloon/Travis Deverell (has a main character that some people find insufferable) and Primal Hunter by Zogarth (a bit edgelord).
Another good place to look is the SPFBO (self-published fantasy blog-off) contest run by Mark Lawrence,
here. The finalists are generally decent. They aren't all on KU, but a good chunk of them are.