Ryanair are pushing themselves as Europe's lowest emissions airlines.
(I can't post URL but here: corporate.ryanair.com slash environment)
Their measure is CO2 Per Pax/km - they say they manage to keep this lower than their competitors because of:
- high load factor (96%)
- young fleet (ave. of 6 years).
So while this makes them efficient they are still probably one of the biggest airline carbon emitters in Europe.