For true quality, there's really no substitute for a well-made leather guitar strap. When it comes to comfort, a wide, padded leather strap is hard to beat.
Some of the most popular leather straps are made by Levy, and are any where from 2 1/2" to 3 1/2" inches wide. They're also padded with foam, and often have a boot leather outer surface, combined with a soft, "garment" leather inner surface.
By almost all accounts, the Levy straps are very well made, and will last as long as most players care to use them. In addition to the boot leather, garment leather combination, they also make an Italian glove leather model, and a soft suede design that is very comfortable. Levy straps are not terribly expensive either, considering their high-quality and leather construction. You can expect to pay anywhere from to about for one of these straps.
Moving up to the boutique level, some of the finest guitar straps currently available are made by Moody Leather. Although they're not cheap, costing anywhere from 0 on up, they're the strap of choice for many famous musicians. Moody Leather is the creation of Allison Suter, who first learned to work leather by creating a deer-leather jacket for her F-15 fighter pilot father, Richard "Moody" Suter, hence the name. The marketing collateral featuring Ms. Suter is not bad either.
If you're looking for something a bit more old school and traditional, you can't go wrong with Franklin Straps. If you close your eyes and picture a traditional leather guitar strap that might be used by Merle Haggard or George Jones, you will most likely imagine something that looks a lot like a Franklin Strap. Franklin Straps are truly beautiful works of art, and come in a wide range of styles, not just the Merle or George look. There's sculpted latigo leather, suede, chrome, graphic and southwest series.
Regardless of what style, material or price level you decide on, it's hard to beat a quality leather strap for your guitar. They're more comfortable, especially for a heavy guitar like a Les Paul or an archtop, they will last almost forever. And, let's face it, they just look damn good.
Good luck and happy playing!
