Inquiry for OS X version recommendation

[LockDown]

I have a few old desktop with almost the same specs and currently running 10.7.5.

 

Core 2 Duo

4GB DDR2 Ram

Geforce 8400/8600 & GT 9500

500GB HDD

 

What do you think is the better version of OS X i can install?

 

I used the term "OS X" cause it might be heavy if i install the newer "macOS"

 

Please note that im not planning to upgrade any parts of these desktop.

 

Thank You!