Unfortunately, there is no "best" shoulder holster. It all depends on what you want, in other words, it's strictly a personal choice.
There are horizontal carry, vertical carry and upside down carry styles. Each style has its pros and cons.
A couple of things to consider. Generally, a wide shoulder harness is more comfortable, and the harness should have an ammo dump pouch or carrier on the off side for balance.
You can start your holster search here:
Defensive Carry Holster Links