Finally, of course, there's the material to consider. Traditionally plectrums were made from natural materials and animal byproducts, including bone, tortoiseshell, steel, amber and wood. The first plastic guitar pick was made by D'Andrea way back in 1922, and this would go on to be the blueprint for the plectrums we use today.
Guitar picks are now primarily made from synthetic materials, including, celluloid, nylon, acrylic, delrin and even glass. Obviously, the material you choose greatly impacts your grip and tone.
Nylon and celluloid plectrums produce a warm, mellow tone perfect for acoustic guitars and 12-string guitars. Acrylic plectrums offer a brighter, more articulate sound, perfect for lead players.
If you are looking for a general pick to do a little of everything, Tortex is a good option. These picks are usually very durable and offer an even sound. There is a reason these are by far the most popular plectrums around.
It is important to remember that these are just guidelines, and there are no hard and fast rules to which is the best guitar pick for you. Experiment and try different things - you never know you may forgo the plectrum altogether and end up using a sixpence, like Brian May!