The best global SIM cards are those that are useful to you. It must cover the places that you intend to go to. Most of these are part of large networks that cover over 200 countries so you are likely to have no worries. You can check their list to be sure.
Compare the rates of these cards for calls, texts, and data. See if they are offering any promo packages that can help you lower your expenses. Are their data allowance good enough for you? Are their partner networks fast and reliable? Do they have strong signal in the part of the country where you intend to stay?
Most will provide you with a US number that you can share with your friends for ease of contact. Others will throw in a European number as well for flexibility. Choose according to your needs.