Well, I work for Atlas. However, I tend to agree with most people here. UA, SW then Atlas.
The only way I’d pick Atlas over any of those is if you absolutely want to fly cargo over pax. If that’s the case there are many order companies to take into consideration as well.
If you are looking for quick upgrade then UA is the way to go. SW upgrade times are crazy high and someone is lying if they tell you 6 years.
At Atlas the 777 is the second most junior airframe, with a 2019 hue to upgrade.
Id say to go for SW, if you are looking for a fun culture.
just one more thing to add to the mix. Try to live close to where you want to be domiciled. Commuting really sucks, and the most junior CA at UA will be in expensive cities. This is where Atlas has a huge benefit, their gateway is really nice. I’m based in JFK, and have had a ton of luck with deadheads on both ends of my patterns, so I’ve been home about 4-5 days more a month.