That's hard to say because they are all relatively different. If you want to be able to process payments with an iPhone or iPad, then PaySimple & Apple Pay would be the best option for you. But if you run an eCommerce business, it might make more sense for you to use Square because credits cards are popular in the United States and they accept ACH deposits. Venmo would be a perfect fit too if your customers frequently split large payments between groups of people who don't have bank accounts. PayPal is great if you're sending money internationally - it also is a good app that can store transactions on your phone so that they're easy to pull up when needed. If security is important, Stripe may be the right option for you because they are the most secure.