With picks, there are 3 main types: soft (for strumming), medium (for stuumming and solo-ing) and hard (mostly soloing). Now, what should you choose for your instrument? It is known that the string type matters more than anything else when choosing the picks that you should use. There are main 2 types of strings: plain (steel, nylon or gut) and wound (which are made up of more than a material). What matters most is the stiffness of the material, meaning that you should not hit soft strings with hard, butsoft picks. For traditional instruments you should use the specific plectrums (another name for picks) and for the not-so-traditional you should take time to see which can help you play best. For country music, where you strum more than solo you have a medium pick, as you most likely play an acoustic. If you solo on a Spanish classical, you can use the same, but make sure that you do not push the strings too much when picking. For heavy metal, the harder the better especially when it comes to the bass, where you can use triangular picks. For the banjo, you can use thumb picks made out of metal, but you had better not use them anywhere else. What is also a really good thing to have on a pick is a really rigid grip, so that the pick does not fall out of your hand. For such things you can attach the pick to your wrist with a soft cord that goes through the pick (check out the Swiss Pick [http://swisspicks.com/] for just 5.99$). The best reviewed all-in-one pick is the hand crafted Plexstrum pick [https://www.plexstrum.com/]. My personal opinin is that you can use a tringular medium pick on anything, as long as you make sure not to ruin nylon strings.