UPS/FDX are best for money, hands down. But QOL depends on your lifestyle, since there's obviously a lot of rede-eye flying to be done. They also don't hire too many pilots so your odds of getting in are slim unless you're an astronaut (not kidding), well connected, or work for an international ACMI/Cargo outfit flying heavies to gain experience.

SWA is a good company, but the pilot group is young and they've mostly filled their business niche so growth is somewhat limited. They are very disciplined about expanding beyond their niche (good thing IMO). You would wait a long time to upgrade there, but would probably have good QOL. Realistically I'd shoot for one of the big three if you stick with 121.