Best OS-X to install

This system can only go up to El Capitan (10.11.x) with that said, Yosemite (10.10.x) has a slightly smaller footprint so you might want to use it if you only have 4 GB of RAM. But, make sure the Apps you are planing on using will run under it (or what ever OS you are planning). FYI: Your system is able to support 6 GB of RAM (one 2GB module and one 4 GB module) per 3rd party testing.

You also want to make sure your systems firmware is up to date before adding any memory (if you are still at Mountain Lion 10.8.x) go to this Apple T/N About EFI and SMC firmware updates for Intel-based Mac computers it will walk you through checking & upgrading.