I'll certainly vote for Southwest- their fleet has to be well-maintained to manage the kind of systemwide reliability that they deliver.
I am acquainted with an individual who performs work for various airlines, including
WN. He tells me that, while most carriers forward assemblies with an "IRAN" tag (Inspect And Repair as Necessary), Southwest directs that all work is to be done to "as-new" standards. Anecdotal, I know, but consistent with my observations.
Perhaps the 737 Doc will pipe up on the issue.
Bear inmind that well-maintained aircraft are reliable; and reliable aircraft fly more; and aircraft that fly more, make more money.
...three miles from BRONS, clear for the ILS one five approach...