The Windsor Knot
If you would like, you may listen to an audio version of this video.
The windsor (also known as the full or double windsor) is what I consider to be the godfather of all the knots. This large knot provides wonderful symmetry and balance in a very classic, official-looking shape. This is generally the knot that is most appropriate for practically any formal occasion. It is somewhat more difficult to tie than other knots and uses a significant amount of fabric, so thinner fabrics may be more appropriate if you don't like the knot to get too big. Along with the four-in-hand and the half windsor, this is one of the three most popular knots.