People can have good and bad anecdotes about domestic F upgrade chances, but of the Legacy US 3, AA is statistically the worst at monetizing domestic F, and I love it. My upgrades as Gold/Platinum cleared at 50%+ chance until I ran out of stickers, and with Platinum Pro it's even better. I think I've sat at the back of the LAX-SJC Eagle only once last year despite making that trip many times. And for all the times the upgrade doesn't clear, MCE seat selection availability is great, and I'm basically only in regular main cabin seats if I think there's a chance I get an empty middle seat that way.
AA makes it the most rewarding to fly internationally on all the foreign carriers I'd rather fly on.
The lack of seatback displays makes it easier to sleep on domestic red eye flights.
Transcon business upgrades work like domestic first upgrades. If only they'd let complimentary upgrades from transcon business to first instead of having a non-rev party up front every time...
The Flagship lounges are legit nice.