Lufthansa is no. 1
I get upgraded all the time, their airport-lounges rock, and the catering is mostly unparalleled.
Otherwise any other Star Alliance member airline, I'm a milage junky!
KLM sucks ass, stupid baggage regulations, unfriendly service, the list goes on.
Otheres on the shitlist, but they don't exist anymore:
Sabena = Such A Bad Experience Never Again