I fly about 100,000 miles a year: over 50,000 on AA, most of the rest on Delta, enough on others to at least get a sense of them. This includes US, Canada, trans-Atlantic and intra-Europe.

IMHO, AA is above average though not at the level of service you get from some of the smaller carriers. I find their coach food the best of any US airline except Alaska. (The Canadian big 2 have them beat by a mile/kilometer.) Domestic first-class and trans-Atlantic business class are average. Service is better than most. (I haven't flown AA into or out of MIA.)

If you ask me, the one that's gone downhill is Delta, which can start another argument... I find that a shame. I used to fly with them whenever possible. American's recent policy change to award Executive Platinum level for 100,000 miles, not just for 100,000 points, will cost Delta the rest of my business.