Well you may find disagreement from others (like Ken Rockwell), but I love my Nikon 40mm f/2.8 micro. You can buy it brand new for about $275. I love it on my D3200. It is very well made, remarkably sharp, and very versatile. It gets some poor press because of its short focal length (which is not that short on a DX camera), and because of its requirement of close proximity to the subject from the lens. Admittedly, if you're trying to photograph insects, this will scare them away. Also, if you're shooting indoor subjects it can be very difficult to light. However, I use mine for outdoor natural light nature photography, mostly flora, and I find it to be perfect. It's also a great, relatively fast general purpose DX lens. The Nikon 105mm micro is probably the ultimate but costs nearly $1,000. For general hobbyist outdoor macro work, my money's on the 40mm. If you're interested I can link you to my Flickr account to show you some samples.
It depends a little on what you need to photograph. The 40mm micro is said to be wonderfully sharp, and versatile, in that it also serves as a very nice general purpose lens. Its macro capability is limited a bit by how close you must come to your subject, but if you have good light and your subject is not a nervous insect or the like, it's fine.
If you want to do bugs and butterflies, you'll wish for a longer lens, but you can do pretty well with a regular telephoto zoom and cropping.
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If you want to do bugs and butterflies, you'll wish for a longer lens, but you can do pretty well with a regular telephoto zoom and cropping.