Can only speak for the US - AA
for network carriers, AirTran for LCCs (although Frontier's is also nice.)
I like DL
's basic design, but it does make some things a little difficult to find, and their stubborn insistance in not allowing searches with the "NYC" code drives me nuts.
JetBlue's is easy to book on, but the route map (despite the recent improvement) is still far inferior to AirTran or Frontier's.