El Capitan is no longer getting security updates from Apple.
Sierra still gets those, but will likely stop when the next macOS is released in about 6 months.
High Sierra might be a good choice, and gets you down the road another couple of years.
But, Mojave is the last macOS system that will still run 32-bit apps (such as MS Office/Word/Excel 2011) some of which I still depend on.
Dark mode is also not my fav, so I don't use it (did modify it so i have a dark dock and top menubar). And, I don't experience any of the kinds of challenges that some others report. Mojave is quite stable for me, being 5 "dot" versions, and seems pretty mature at this point.
So, I would recommend going to Mojave at this point (unless you have an iMac with just a spinning hard drive, which will prove to be a bad choice for Mojave. Even a fusion drive is better, with best choice being an internal SSD.