OSX 10.11 El Capitan

Back up your data, check your applications for compatibility, and click on El Capitan.

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FWIW, El Capitan will have Safari certificate problems. macOS Sierra 10.12 or (preferably) macOS High Sierra 10.13 or later will avoid that mess. Either of the MacBook Pro Mid 2012 models will support Catalina; macOS 10.15, though Mojave will avoid having to check for and retire 32-bit apps.

You want High Sierra, if your Mac can get there.

The oldest macOS will Microsoft Office support is now Catalina, for instance.

Performance-wise, I haven’t noticed an appreciable difference between El Capitan, Sierra, and High Sierra. Or with Mojave, for that matter.

Catalina, Big Sur, and Monterey have all made more significant demands of the hardware, so those did get slower on older low-spec hardware. Catalina and later particularly don’t run well on configurations with eight gigabytes of memory and a hard disk drive, or eight gigabytes and a Fusion drive. More memory somewhat masks the slower HDD. On systems with SSDs, those versions do better, even with eight GB.