Best of best OS for iMac 27 late 2012 ???

[Racineur]

Hello, my old late 2012 iMac 27 (3,2 i5, 1T FD, 24 G Ram) can boot with these OS: Mountain Lion on the internal HD, El Capitan, Sierra and High Sierra on external SSD. Mountain Lion seems runs very very fine but some apps like Safari and Mail sometimes can't cope with 2018. El Capitan seems the best bet. Sierra is a little demanding on the ressources (more Beach Ball, a little stuttering in opening windows, etc). High Sierra is very demanding. Opening and closing windows, using Mission Control crates choppiness. Plus, iStats shows the strain on the NVIDIA GeForce GTX 675MX. The 1024 Mo memory is always near full. Sierra and High Sierra make tie iMac warmer too. I know it's an old machine but it still fulfills my needs. What is your take on that. I figured out El Capitan is the best but maybe I get il all wrong.
Thanks a lot

 

[baas]

El capitan is very solid and stable release, but it will probably stop receiving security updates this fall. All versions bellow El cap (like Mountain Lion) don't receive them as well so I wouldn't install those. You could install El capitan if that's what you like and upgrade to Mojave when that's available or go with Sierra and wait till Mojave got a couple .x releases.

 

[dogslobber]

El capitan is very solid and stable release, but it will probably stop receiving security updates this fall. All versions bellow El cap (like Mountain Lion) don't receive them as well so I wouldn't install those.

I was also wondering about when El Cap will stop being supported. It feels like El Cap is the modern equivalent of SL so may get an extra run of security patches. Perhaps wishful thinking from me.