The latest macOS that can be installed on your MacBook Pro A1278 (Mid-2012) is macOS Big Sur

 While macOS Mojave (10.14) is also a good option.

You will get 

OSX: macOS Mojave or higher

Model: MacBook Pro A1278 (Mid-2012)

Processor: 2.5 GHz Core i5

RAM: 8 GB (upgradable to 16 GB)

Storage: 250 GB HDD