Frontier used to be a pretty good airline. But that was before they switched to the ULCC model. Now, they are completely off my list.

Do you fly enough to get meaningful elite status? If not, then Southwest is definitely worth a look.

If you do have elite status then United will almost certainly be the airline that will get you where you are going with the fewest transfers.

Or stick with Delta. You'll likely be transferring a lot in SLC, ATL, and DTW, but I think it is the best overall domestic carrier.