As some others have said, Crosby at 8.7m through his entire career may be even better...
Even at this age, it's a pretty good value. He consistently has gone with the Tom Brady approach and took much less than what he could have to allow the team to build around him. (It's part of what's insulting at this current point of the way the club is managed).
His on-ice impact is apparent, but off the ice may be even more beneficial. He almost single-handedly saved the franchise, revitalized hockey interest in Pittsburgh, and was the face of the league for a while.
He could have demanded the keys to the city and it would have been an underpayment.
Edit: That's not to say Marchand's contract isn't great, but for what Sid provides as a total package it's hard to beat for the $8.7m price tag.