Canon's 100mm macro is the most "Bang for your buck." There's also the 60mm macro, but I have been pretty unimpressed with that lens, having used both. I haven't, however, used the 180mm macro. While it's supposed to take the most phenomenal shots, it also comes at a phenomenally high price.
These are just Canon's macro lenses. I'm a Canon snob, I admit. Sigma and Tamron also make macro lenses, but I've never bothered to consider them. Canon's glass has an awesome reputation for a reason.