Books

The closest I've got is Neal Stephenson, specifically his System of the World trilogy vs the standalone Cryptonomicon, both set in the same larger context, and even with a character that shows up in both.

I loved the standalone, but I've tried and failed multiple times to even make it through the first of the trilogy.

The Expanse novellas (if that's the kind of thing you mean) are all quite good, and expand on stuff that wouldn't have fit neatly into the main series' pacing/structure. The show even manages to weave them in directly.

The same thought crosses my mind that these books are probably not as good as the series of books and are more world building or cashing in on the success of the others.

If quality is your concern, there are plenty of series that have less-good main entries, or are bloated with stuff not everyone will care about, or are like 40+ books long but the same plot every time. Personally, I'd rather have a solid, streamlined series with side content available for those who really love it and want more.

Brandon Sanderson has lots of stand alone novels that are just as good, or better than the ones that are part of a series

My challenge with giving a meaningful input to this thread is that I struggle to think of an author whose books series I have read and have read their stand alone stuff.

The closest I can come up with is Leigh Bardugo and this answer doesn’t really fit what you are asking for.

Her main series is the Shadow and Bone trilogy, her stand alone book that actually became a duology is The Six of Crows, which I enjoyed much more than the Shadow And Bone stuff. In fact I couldn’t finished the second book of Shadow and Bone.

I've read most of Clive Barker's Books of Blood stories and don't like any of them, even though they're probably his most recommended works. His novels are all worth reading, and some of them are quite fantastic. I particularly love Imajica and Coldheart Canyon.