What is the best upgrade for my imac OS from High sierra 10.13.6? I have 32GB of memory (8 in each slot). 64 available.

Whether, and how far, you can upgrade macOS depends on which Mac model you have. If you look in About This Mac in the Apple () menu, what does it say about the model name and hardware model year / time of year?

+1 we cannot answer the question until we know what model year and size iMac you own. However if you are trying to make the Mac faster RAM has an extremely minor improvement and generally is not worth the cost!