I'm yet to hear of a bad macro lens. They are typically among the sharpest lenses of any manufacturer. I usually say that around 90-105mm is probably the best focal length for general purpose macro photography. It's basically an all round length. It gives you a bit of working distance (although more is better) if shooting insects, but isn't too long to make botanical macro work awkward, particularly in more confined spaces. However, if you are only intending to photograph flowers, then it's probably also worth considering a shorter focal length. For one thing they are less expensive, but also, some gardens aren't particularly large, so you may be limited on space. That's ok for 1:1 macro work, but if you need to back up to photograph a larger area, to produce a closeup, rather than a true macro, then you may find yourself running out of room with around 100mm. As AJ said, think about the areas you are likely to be photographing in and factor in any potential secondary uses.
