I think it is a very close match up between the two.
Backstrom is a very underrated center, oft overlooked because of his connection to ovy, however a lot of the time this year (excluding power play) he didn't play with ovy and still put up great numbers. Kuzy on the other hand is probably the craftiest of the bunch but also has shown that he doesn't always dominate the game. When he's on, he's on tho.
Matthews is fantastic, highest potential put of the 4, already the best goal scorer of the 4 as well. Consistent but has not reached the level that most people hype him at in my honest opinion... The biggest question, and possibly overlooked thing for Toronto centers is Tavares, he's coming into a brand new situation. Yes he put up those points last year with barzal being a dynamo but I don't think anyone looked at him as anything but the best player on the team. That being said, the question mark is how many points will he put up splitting the best wingers with Matthews?
I believe that Matthews and Tavares are better on paper than backstrom and kuzy.. but Backstrom and Kuzy played together and have cemented themselves as worthy to be talked about in the same breath as Matthews and Tavares, and untill I am able to see how two goal scoring centerman play together, I'm going to give it to Washington's duo for now.