My personal fave is Grassroots Motorsports because they're the best source for HONEST opinions on the way a car performs. They're also a shoestring operation, so they're not restricted by advertising bucks and/or the interests of a big publishing company.

However, I'll admit a little bias myself because I'm an autocrosser, and I tend to judge cars largely by their ability to get good lap times The mainstream car mags tend to talk about handling in vague generalizations, and rely too much on slalom and skidpad numbers. The other problem with GRM is that they're such a shoestring operation that they have trouble getting carmakers to loan them test vehicles, so they don't test a whole lot of cars