My Crystal Ball:
If you are asking which regional will grow the most over the next 5-7 years (likely the time to get hired at a major), you are probably asking the wrong question.
As big retirement backfills hit with the majors, the sucking sound out of the regionals will make it hard for them to keep up.
I predict several of the regionals will fold up their tent and go away. Others will merge. So in that time there will be significantly fewer regionals. I would even go so far as to predict more flying will be done by the majors.
This is simply because 76 seat routes will get upgraded to 100 seat at the majors and many 50 seat will upgrade to 76 seat. A lot of the 50 seat metal will be parked. This eases the demand for pilots needed just a bit; bigger planes, less number of pilots to fly the same PAX numbers.
Your question should be which regional will be around long enough for you to get hired at a major? (I suggest avoiding dumpster fires.) Where are you going to live; hopefully at a regional domicile (or second choice easy commute)? Do you want to be at a regional with guaranteed flow or interview?
Or, if you want to stay with a regional and never go to a major, where are you living vs. domiciles, and which are strong enough and stable enough to continue to exist. And I would ask why wouldn't you want to go to a major? Most lifers at regionals, if they had it to do over again would now jump at the chance to go to a major (assuming they have enough runway of years in front of them to make it worthwhile.)
If you are asking about the next 2 years, growth is a valid question, but that is just one of the factors to consider for upgrade to CA. But that has little bearing on when one can get hired at a major.
Just my opinions. YMMV