The best kind of telescope really depends on what you want to use it for. There are two basic types of telescopes, refracting and reflecting. Refracting telescopes use lenses to focus the light, and reflecting telescopes use mirrors. I'll talk first about refracting ones.
Refracting telescopes work by using two lenses to focus the light and make it look like the object is closer to you than it really is. It takes two lenses to do this, and both of them affect the total power of the telescope. The problem with this type of telescope is that it requires both lenses to be held up right at the edges. The bigger the telescope is, then the bigger the lenses must be. Since big lenses are really heavy, they're very difficult to support just by the edges, so it's hard to make refracting telescopes that are very big. The biggest refracting telescope in the world is located at the Yerkes Observatory of the University of Chicago at Williams Bay, Wisconsin. It has an objective lens that's 1.02 meters (40 inches) across, and it's total power is 195x.
(The 40 inch telescope at Yerkes Observatory)
But the total magnifying power of a refracting telescope is not the only thing that's important. I mentioned that both lenses work together to determine the telescope's total power. This means that it's actually possible to use different combinations of lenses to get the same power telescope. But the objective lense determines how much light is able to get into the telescope at all. The bigger the objective lense is, then the more light will be able to get in, and the clearer the image will be. So if you're trying to pick the best refracting telescope, it's important to get not only one with a high power, so that the image will be enlarged a lot, but also one with a large objective lens, so that the you will be able to see the details of the image clearly.
Simple reflecting mirrors, on the other hand, have to be big in order to do much good. In order to see the image, the person has to be sitting right at the focal point. As you can see from the picture above, this means that the person is right in the way of where the light is coming in. But if the mirrors are really really big, the person can sit there without blocking very much of the light. And fortunately, since light doesn't have to actually go through the mirror, it's very easy to support one that's really big.
It is possible to make a smaller reflecting telescope by using a tiny flat mirror located at the focal point to reflect the image out of the side of the telescope tube. A small reflecting telescope will generally have a larger magnifying power than a refracting one of the same size, but small refracting ones are much easier to make and also much cheaper. But if they can afford it, scientists tend to prefer to use reflecting telescopes for research.
(The reflecting telescope built in the 1780's by the famous astronomer William Hershel)
There are also a wide variety of types of specialized telescopes that use both lenses and mirrors, but these are usually extremely expensive and are more difficult to use.
(republished on 07/29/06)