I think the Mazda3 is a great car. It's strikes a really good balance to make a great drivers car. I can see why car and driver picked it. Honda had decided to target the Civic to a more general population who wants a lot of technology integration, mpg, and a luxury feel. Driving it home tonight I gotta say the Civic is no slouch in the corners.
Mazda3 IMO handling is quite tight which gives it a sporty feeling especially the 2.5L, however the features/packages are quite limited and it ends up going over 30k to get some of the AWESOME features of '16 Civic - some Civic features are not even offered in Mazda3 even at the highest trim level...
Civic has been positioned as a well balanced car for several years, now with the 1.5T and aggressive design... it's going back to the Civic roots which is attracting the younger gen more than ever!
With the Accord likely growing for the next generation and the Civic having grown for this one, I can see Civic being named a Top 10 in place of the next Accord. Like how the BMW M235i replaced the 3 series last year and this one because the 3 series grew and no longer handles like it once did. C&D seems to like smaller more fun cars for this list.