We have seen:
Water vs Water (Pakku and Katara)
Water vs Earth (though no good ones)
Water vs Fire (Katara and Zuko)
Earth vs Earth (Earth Rumble 6)
Earth vs Fire ( Tons of examples)
Earth vs Air (Aang and Bumi)
Fire vs Fire (Zuko and Zhao, Zuko and Azula)
Fire vs Air (Zuko/Azula vs Aang)
but we have never seen an airbender fight a waterbender. I mean amon fought korra but he was using bloodbending and it was uninteresting. but we have never actually seen water vs air
(we also have not seen air vs air)