iMac (21.5-inch, mid 2011) What software version to upgrade to?

Hi 

I have a iMac (21.5-inch, mid 2011)

 Model Identifier: Mac12,1

 Processor Name: Intel Core i5

 Processor Speed: 2.5 GHz

 Number of Processors: 1

 Total Number of Cores: 4

 L2 Cache (per Core): 256 KB

 L3 Cache: 6MB

 Memory: 12 GB

Graphics: AMD Radeon HD 6750M 512 MB

This mac (High Sierra i have )

What is the most recent and best OS version that I should upgrade this iMac to?

Please advise. This mac (High Sierra i have )

Many thanks.

Cahit