Start with a check in that's as hassle free as possible. My recent experiences say that's BA at LHR, LGW, DFW and PHX and Servisair as the BA agents at CPH and ARN. Check in with AA at DAY was a nightmare, arguing with the woman that my bags were to be checked through to ARN, as that was where I was going, and explaining that it was no use saying I couldn't be booked through as the flight had already left - she was trying to book me on to that day's LHR to ARN instead of the following days flight!: America West at COS not much better.
Competent security. Not too impressed with the US, although I think it's noticeably better now that the TSA people are there. Even though at COS the other week, they unloaded my suit case and came running down to the gate to give me the pair of shoes they'd forgotten to put back in the case!!!
Security at LHR and LGW always strike me as OK: ARN terminal 2 as the best anywhere.
Decent lounge for frequent flyers. BA wins hands down here, although the lounge at Nice (Servisair?)isn't bad. ARN Finnair lounge (shared by the One World people) is a bit basic - even the section for the BA Gold card holder.
Cabin crew that at least appear pleased to have you on board. Even in Business class transatlantic on American, this doesn't seem to be the case. However, AA have had that many lay offs I suppose it's hardly surprising.
Pet HATE: (confined mainly to the US) Safety briefings gabbled through at high speed, often at too low a volume, probably on the basis that few people listen to them. Sometimes in an incomprehensible accent, too. The commuter flights seem to be the worst, although I've had a couple of bad ones (again on American) on mainstream flights.
Another hate is all the drinks coming at the same time as the meal. I know it's not always avoidable, especially on short flights, but longer ones can be better dealt with. Not so easily, I suppose when BA cuts the number of cabin crew..........
Getting the balance between being too quick with clearing away the meal service and too slow. Very difficult.
I'm probably spoilt by mainly having used BA Club and Club Europe for the past 17 months (32 round trips, 73 flights). When BA CC have an off day, (which, let's face it, everyone does occasionally) my experience is that they are only very good. Most times they are excellent. Other airlines I've flown in the past year (SAS, American, Finnair, South West, America West) have all given the impression to a greater or lesser extent that they'd rather not be bothered with passengers.