Best version of macOS to run on my 2009 MacBook Pro

I guess Firefox 78.15.0esr is the best browser for OS X 10.11.6 El Capitan.

https://ftp.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/

...in El Capitan I use also Carbon Copy Cloner 5.1.28.6213, BBEdit 12.1.6, KeePassX 0.44, LibreOffice 7.2.7.2.

El Capitan 10.11.6 is the most up-to-date version of Mac OSX that you can run on that computer, I believe.

As far as Internet browsers, as Matti Haveri says above, Firefox works much better than Safari in El Capitan, so is much preferred. I also run 10.11.6 and have found that an obscure browser called Pale Moon actually runs faster than Firefox on some websites, but on other websites Firefox is better. Try both, see which works best for you.

OS X 10.11 is the latest available version for that.

That’s ~ten years back, with yearly releases.

Linux and BSD will probably also boot on that Mac.