I like C&D, it provides many real-world car tests, stuff that the majority of people buy, and its good to know about those cars (helps when I advise just about everyone what to buy). THey also have some good car reviews, and I always enjoy reading it.
R&T gives me lots of racing news, and they have a lot of techno-babble that I love reading. They do quite a few neat car tests, too.
MT does lots of super-fast car tests, lots of speed, which is nice to see.
I feel all magazines have their strengths and weaknesses, so buy them all!!