Probably the A300 or A320; sadly, after those they've a history of conservatism.
The A380 was the once in a lifetime opportunity to move airliner design on by a generation, but they lacked the corporate courage to do so and just made a bigger 747. Yes, impressive engineering in isolation but quickly surpassed in economics by subsequent tube-and-wing designs with better engines. What was the point? Didn't even break even on the project. It was obsolete as soon as it entered service.
The debacle of their response to the 787 showed their lack of decisiveness and when they finally launched the XWB it was ... narrower than 1968 designs for the A300!
The Neo designs just rely on engine manufacturers getting better at their work, while Airbus does very little.
It's a mercy that the Sonic Cruiser never went into production because Airbus today is too risk-averse to have had any chance of responding to it. They've become the lazy incumbent they disrupted in the 1970s.
