A 180mm macro is a great choice for this reason, but they are slower and more difficult to hand-hold. I chose the 100mm as a compromise. This lens features internal focusing, and importantly the lens does not increase in physical length as one gets closer. It's worth noting that at macro focusing distances autofocus becomes more of a hindrance than a benefit. Turn it off and use manual focus, and if you're hand holding use the focus ring to establish magnification and then actually focus by moving back and forth slightly.
Depth of field is of course minimal so a tripod is usually called for.
Frankly, it makes doing macro nature photography a breeze, especially with a Canon EOS 1V , EOS-3 or D30 each of which is able to use the flash's full capabilities. A ring-lite allows you to illuminate small objects with shadowless flash, even at the closest possible distances. The MR-14EX has some unique capabilities that bear mentioning. Firstly, the ring has separate left and right flash tubes, and the light level can be set independently for each up to a 6-stop range. This permits creating modeled lighting effects.