You pretty much have to decide what type of photos you want. If it's a bee sitting on a flower as Brian's gorgeous photos show, then you want a macro like the Tamron 90 or the Nikon 105.
But if you want to walk around shooting nature photos and sometimes get in close like these, then maybe a close focusing zoom is in order.
Or maybe a grouping.
And then maybe a scene you like but are limited by how close or how far away you're allowed to get.
All of these photos were taken with the same lens, the AFS 18-200mm zoom, and are not cropped or sharpened.
I believe the first 4 photos were taken with the D200 and the last 2 the D2X.
edit. I'm very fortunate to live close to Longwood Gardens in SE Pennsylvania. I try to visit during each of the seasons. More than a few times I've only carried one lens, the 18-200mm. On other visits I've carried a macro, a zoom and maybe a fisheye. But that bag gets heavy after a couple hours of walking around.