In terms of creativity, Icelandair has been the most impressive. When airports throughout Europe were closed, Iceland's main airport in Keflavik remained open. From there, Icelandair operated a strategic mission every day. One day, Trondheim airport in way northern Norway opened, so they sent the fleet over there with several flights. Another day, they thought Heathrow would open so they scheduled the flights. When it didn't, they sent the planes to Edinburgh instead. I would say that Icelandair probably served its passengers better than anyone.
But after the volcano threat died down in Europe, shifting winds actually shut Keflavik for a few days. Uh oh, that's the end of their work, right? Not so fast. Instead of just calling it quits, Icelandair opened a makeshift hub in Glasgow. Flights from the US went to Glasgow, where passengers could then connect to other parts of Europe. They even sent flights from Akureyri on the other side of Iceland to Glasgow so people could get to their destinations, if they were willing to endure a bus ride from Reykjavik.
The whole time this was happening, Icelandair made masterful use of Twitter and kept Icelandair.com up to date for all stranded travelers. For some people from the US who weren't able to get to their destination in Europe, Icelandair provided a first night's hotel and meals in Iceland.
Of course, it's a lot easier for little Icelandair and its fleet of fewer than 20 airplanes to have this kind of flexibility than a behemoth like Air France/KLM, but it's impressive nonetheless. Air France operated a few flights out of Toulouse and Marseilles while British Airways tried to run a handful in Glasgow, but really, none of those efforts were even close to what Icelandair was able to accomplish.
People may resent Iceland and its unruly volcano right now, but they should have a lot of love for Icelandair.