unless someone knows otherwise, the most goals that have been scored during a 5 minute power play was 4, and it was by the Toronto Maple Leafs against the Atlanta Thrashers on Jan 7 2011.
On a side note, the Los Angeles Kings have the most famous multiple goal power play. It happened during game 6 of the 2012 Stanley Cup Finals against the New Jersey Devils. Although they only scored 3 goals during this power play, it was a major turning of the tide for LA, as they had lost the previous 2 games to New Jersey. It was that power play that lead the Kings to their first Stanley Cup win!
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the New york suckers
Detroit Red Wings Beat the ranger 15-0 in the largest shutout in NHL history on Jan 23rd 1944. I don't know if that's the most unanswered overall though.