For a standard collar, you cannot go wrong with 3-ply, 2" wide nylon. It is durable, easy to clean, and dirt cheap. I use these as yard collars, and the best supplier is without a doubt Stillwater Kennels,
www.stillwaterkennelsupply.com
If you are looking for something more unique, Collar Mania offers strong, durable collars designed with pit bulls in mind.
www.collarmania.com
While Stillwater and Collar Mania are the only two companies I use personally, I've had my eyes on these for a while:
http://www.toughcollars.net/id16.html (Their spiked collars are too extreme, but if you're looking for a 3" wide collar, these look pretty decent. Another member here reccomended them to me)
http://www.hofero.cz/obojky.htm (Based in Poland, and their site is in their native tongue. They do speak English, however. Not sure if they ship stateside, but I'm having a friend of mine in Slovenia hook me up. Can't wait, these collars look awesome!)