I have flown on all of the big 6, Southwest & Midwest. Concentrating on just the big 6, I would rank them the following way:

1. AA
2. UA
3. US
4. NW . . . .

99. CO/DL

#1 & #2 are in the finish.

AA is better for me because of better upgrades. No H fare worries on using miles. Upgrade on any AA published fare using miles. I had no status & was able to upgrade a cheap fare using miles.

UA is fine. Top tier SWU's upgrades (1K's) started out quite restrictive but UA sent out 4 much less restrictive SWU's. Less miles needed to go to Australia on an award.

#3 & #4: US & NW has their advantages. US has limited overseas routes but lower tiers can upgrade transtlantic routes using modestly restrictive SWU's. Most except for lowest fares can be upgraded. My experience with US is Silver (25K level), transatlantic upgrades only clear at the gate, if they do. Gold domestic upgrades cleared decently. Transtlantic upgrades as a Gold cleared ahead of time & at the gate. As Chairman's, transatlantic upgrades usually cleared in advance.
NW: I have read the Gold/Plat members clear domestic upgrades pretty well. Intl, that's another matter.

CO/DL: Why did I select them as #98,99? Try 50% EQM for low fares, with CO intl, the lottery on upgrades, etc. CO could be better because of the domestic upgrades. Read the CO & DL boards, I flew both of them alot in the early/mid 1990's. Good then, changed for the worse after that.

Bottom line: AA is #1 with me because it meets my needs best. UA comes in a fairly close second.