An essential tool for most electric guitarist, and many acoustic players as well, is the guitar pick. There´s almost as many preferred picks as there are players out there, and it all comes down to taste in the end. But I´ve made a few guidelines, without any "rules", that might help you out when it comes to decide which pick to use:
1. Thickness: The thickness is a topic which is discussed frequently when guitarists meets. "You can´t play fast with a pick that soft" is a common statement. That is a truth with modifications. You don´t need a superthick pick to be able to play really fast alternate picking, but you can´t rely on a superthin pick either. It mostly comes down to you being able to handle your kind of pick well, but I wouldn´t recommend a pick that´s thinner than 0.73 mm. And to be able to play really fast alternate picking, we´re talking Yngwie-fast, you probably would be able to be more precise and tight with a pick that´s at least around 1 mm thick. But you don´t need a 3 mm thick pick to play fast, and to get one of these isn't a guarantee for you to play fast. And while on the subject, there´s no guarantee that your acoustic guitar will sound great while strummed with your softest pick - it all depends on you. If you can handle your pick well chances are that most guitars and most styles will sound good. Try out some different picks, thick and thin, in different styles, then settle on the one you seem to be able to play well with. And when you´ve found the right thickness, don´t mess around. Instead, try to get as good as possible with your choice of pick.
2. Shape: There´s the standard shape, but there´s also some other shapes of picks. For example, the one that Kerry King of Slayer plays with is a triangle, where every side is identical. King likes it this way because if he damages one side of the pick during a song, he can flip it and play on with one of the other sides of the pick. I´ve seen some guitarists that uses coins instead of picks, round ones that are really thick. And there´s more shapes of course, like the sharkfin (which at least looks pretty cool), teardrop and the Tenacious D "Pick Of Destiny" devilheaded pick. So if you´re not satisfied with the standard shape, check out one of the more crazy ones. Maybe one of these will be the guitar pick for you!
3: Material: There´s also a variety of materials used in the guitar pick industry. Nylon, wood, plastic, stone, glass and even carbon fiber is used to produce different types of picks. Try out a couple of these materials and you´ll find the one that´s right for you. Of course they all will provide different attack and sound from your instrument, specifically on an acoustic but also on electric guitars. Back in the day pick makers often used tortoise shell, but of course this material was both rare and expensive. And a bit weak, as tortoise shell picks could easily break. But later on, cellulose would prove that it was just as good as tortoise tonalwise, and even better when it came to durability. Good news for the turtles! Also, cellulose is a cheap material, so this was also good news for the customers. So if you´re thinking of going back to the roots and get one of those tortoise shell picks, think again. It´s better for both your wallet and for the turtles if you stay away from their shells!
4: Special picks: Some musical genres comes with their own picks. For instance many fingerpickers prefers a thumbpick instead of their thumb nail. The thumbpick gives a little different feel and hand positioning than playing with your thumb, but the result is often sweet. So if you want to play fingerpicking in the classic folk/country-style, try out the thumbpick! Gypsy Jazz is another genre where the player requires a thick pick to really get the tone that the style demands. A typical pick for this kind of style is the Wegen Gypsy Jazz Guitar Pick (which is 3.5 mm thick!), but there´s players in this genre that uses different kind of picks. The most important thing is that the pick can provide you with the possibility of playing really heavy. No-one really knows what pick Django Reinhardt, the grandfather of Gypsy Jazz guitar, used but it´s believed to be a thick one with a really worn down edge.
<---The Wegen Gypsy Jazz Pick
That said, many genres has a typical pick, like the red jazz pick, but this often comes down to tradition or some famous guitarist that used a certain pick back in the day. To play in this style you´re seldom forced to use the same pick as everybody else, you should use the pick that you are comfortable with. Now go out there and dig down in your strings with your favourite pick!
The Dava Control Pick, my favourite --->