If My iMac Is Running On OS X Mountain Lion (10.8.5), Which Is The Best OS X To Upgrade To Without Slowing Down My Computer?

I acquired am iMac from a deceased family member and wiped it clean. When I reinstalled the OS that came with the system, it was running on OS X Mountain Lion (10.8.5) and I'd like to try to upgrade the OS. What's the best choice in terms of system compatibility? I know it could operate on El Capitan because that's what it was on before I wiped it but I wasn't sure if that was the best/most recent OS it could update to before things slowed down or compatibility issues arose. Ideally, I don't want to slow down the system. And I want to make sure my apps are compatible -- right now there's no iPhoto on the iMac and it's telling me I need to download iPhoto or Aperture and neither are appearing in the App Store since they aren't supported apps in the newer OS. Any appropriate recommendation would be appreciated!

Thanks!