If I had to choose just one focal length, it would be around 100 mm, but a lot depends on what you intend to shoot. 100 mm is a good all round length for general-purpose macro macrophotography.
One responded claimed that a tripod was a necessity – I don't know how one could take insects on a tripod, unless they were dead. I use a tripod for flowers, but still use flash. The slightest breeze is enough to cause blur so I need additional light to get a decent shutter speed and sufficient dof.
The EF 50 mm f/2.5 I find I only use for copy stand type work nowadays.
I prefer the EF-S 60 mm for flora and the occasional portrait. The 100 mm is too long for whole plants and I find the 60 mm lens reduces the need for lens changes.
The 100 mm f/2.8 is a great general-purpose macro lens. I use it for small flowers, flower parts, insects, and etc.
For flying insects such as damsel, dragon, and butterflies, I which I had more working distance than the 100 mm gives me. But these insects are much larger than the sensor so 1:1 isn't necessary. A good close focusing telephoto lens, any good tele with an extension tube, or perhaps a good tele-zoom maybe better for these.
The 150 mm Sigma, is by all accounts a excellent lens, but it doesn't have much more working distance than the 100 mm canon.
Nikon has a 200 mm macro lens, although I don't know whether it is still in production.
For example, the 100MM F2.8 macro is a great lens,
but w/a 1.6 crops that almost a telephoto
- might be tough to hold on a small object.
At close focusing distances, i.e. at or near 1:1, there is no noticeable difference between handholding a 60 mm lens and a 100 mm lens. The lateral components of handshake have a much greater impact than do the rotational (angular) ones. At normal distances camera shake is magnified by the narrow angle of telephoto lenses, this is not so true at close distances.
Brian A.
