I think it depends a little on your style and your interests. Fitness can still be a broad topic. I would go somewhere with a good selection and peruse a few before subscribing.
The best magazine I have found is Women's Health. I'm very impressed by their range of topics from fitness, nutrition, and general health. It has a great lay-out and it is totally worth it. My husband used to subscribe to Men's Health but he found more useful info in Women's Health (although some of it didn't really pertain to him ) believe it or not and now we both read it.
I find Oxygen the absolutely best (and I like Men's Health mag as well, and in fact, I am now a subscriber. I think it totally beats the women's fitness magazine except for the silly advice "What Turns Her On" etc.
I agree with ennay - go to big store like Chapters and check out a number of magazines. I am sure you will find something that you suits your own interests.