I don't really have a favourite as such any more. Nowadays, to me, it's more about favourite flights, regardless of the airline. That is, a flight where the service was good (in my experience, they're all much of a muchness these days, although whether I have to pay for drinks and snacks or not makes a huge difference) and I have enjoyed the overall experience thanks to a friendly crew, comfortable aircraft etc. I've found that you can have a great flight with a particular airline one day, then fly them again a week later and they're awful.
I used to say that Virgin Atlantic were my favourite, but after a ropy experience with them recently, I'm no longer so sure. On the other hand, I may just have caught them on an off day. I've had a couple of great flights with Air Canada, but from what I've read on here, others haven't been quite so lucky. I enjoyed my flights on Icelandair, others, it seems, didn't. I've encountered lovely, friendly F/A's on Aer Lingus, although others have disagreed. My list goes on...
Anyway, if you held a gun to my head and made me pick a favourite, I'd have to say British Airways. I guess you always know what you're going to get with them (right down to the pizza on European flights!).
Ladies & gentlemen this is Captain Tobias Wilcock welcoming you aboard Coconut Airways flight 372 to Bridgetown Barb