Best lens for the Moon?

When I Try taking a photo of the moon it turns out small with a bright dot. It looks nothing like the moon so what would be the bet lens to take a clear crisp shot of it? I have the nikon d5100 with the 18-55mm lens

You need at least 300mm to get a decent shot of it.

You need a lens of around 2600mm in 35mm format to fill the frame with the Moon, for your camera with it's 1.5X crop factor you need around a 1650mm lens.

Here's a video I've just posted of the Moon taken with my GH2 with an old 500mm lens, the GH2 has a crop factor of two, so the lens performs as a 1000mm lens and the GH2 can crop the middle 2Mp out of the centre of the sensor giving me another 2.6X which is the required 2600mm in full frame 35mm terms.

The movement you see is the actual speed the Moon moves across the sky, it's own diameter every 3 minutes.

The cheapest way is to let a telescope project onto your sensor, Google 'digiscoping'.

The Moon this large is as bright as day 1/125th @ f8 and an ISO of 100 is about right.

You can get a decent shot with a 70-200 if you crop it after the fact of course. I shot the moon this morning at 200mm it turned out pretty cool, defiantly not filling the frame or anything, but you can see all the details.
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I shot this with my 75-300 hand held on my D300s. Equiv 450mm I could have put a doubler on it to make it a 900mm but I think this works.

To get the moon full frame you would need a focal length of nearly 2400mm. On your smaller sensor, it would still require a focal length of 1500mm.

That is a big *** lens. The Canon 800mm is $14, 000, plus another $400 for the 2X teleconverter.

There are some less expensive options.

First you can try renting a lens. Make sure you have a high quality tripod, because those long lenses weigh nearly 20lbs, and will crush a tripod from walmart.

Second, you can crop the image down. You will still need a longer lens, but if you had a 300mm lens, with a 2X teleconverter on your camera, that would give you a 35mm equivalent focal length of 900mm. A 900mm lens will create an image where the moon takes up about 1/3 of the frame. If you cropped that image you would be left with an 8MP image where the moon takes up 2/3 of the frame. That would still be good for most applications.