I'd split my response into two -

Best Airline for flying
It's British Airways, warts and all.
Best on board product in all classes and best lounges (In the UK)

Best Airline for redemption
That's somewhat different
BA is stingy and often poor availability even for Gold cards.
BMI is more generous and availability seems to be better. Redeeming BMI points on Virgin for example is fairly straightforward although by phone.

I wouldn't count on any BMI Regional-US services being maintained.

Hope that helps.