There is no best for everyone. What is best for one person may not be best for another. I would go to the "second best" regional in a heartbeat if they had a base where I lived and the "best" one didn't.

The better regionals to be at in terms of how the company treats the pilots and stability are usually the ones with the really long upgrades because people don't leave. The ones with high turnover and quick movement usually have high turnover for a reason, but some people are willing to gamble on chasing upgrades. Other people aren't.

There are too many factors to come up with any one "best" regional unless you list some of the things that are important to you. Do you want a quick upgrade? Best work rules? Best pay scale? Flow to a major? Where do you want to be based? If you list what you want then someone can help you out.