Do check out the Sigmas, either the 105 or 150. I have the old 150 and it is a very good performer. I went for the 150 over the 105 because of the extra working distance which I find handy when shooting bees, butterflies etc. Tos see some examples look at my flickr page and search for Sigma 150/2.8 as I always tag my photos with the lens used.
The best would probably be a Nikon 200mm f/4 or the Sigma 180mm f/2.8 APO or even a Nikon 70-180mm, but they are expensive. Look at the older Nikon 105mm f/2.8 D version, it can be had for about £250 quid or even the Tamron 90mm gets decent reviews.
I had the Sigma 180mm F4 Macro, a big old beast and very unwieldy, but great for getting distance from bugs and small creatures. Sadly sold it as I wasn't using it enough and decided to cash in.
Macro lenses seem to be universally good, bang for bucks wise the tamron 90mm or the sigma 105mm are excellent options. AF speed wise they're not all that bad provided you have one with a focus limiter (both the ones I mention do)
I have been using a Sigma 105mmF2.8D which is the old version and can get decent results with it, but a ring flash is needed to get uber results, so keep that in mind (I am yet to buy one, but have been blessed with a heat-wave)
I had one of these and sold it to another member on here and regretted it ever since.
It's a great prime for portraits and other stuff as well as macro.