Your favorite payment system for a SAAS web app?

What's the easiest payment/billing/subscription system to set up with a Saas web app without much effort? I have validated a product idea with some traction now and looking to monetize one of the features. Stripe, Square, and Paypal come to mind but I am in my initial research and looking for any other attention-worthy recommendations from this community. Would like it to handle baseline use cases: like cancellations refunds payment reminders credit card one-time annual payment monthly payment change of plan pro-rate In terms of UX: The user sees the upgrade button, is taken to the billing app, selects/pays the monthly or annual plan and is returned to the app, and has access to the paid feature now. Appreciate the help as always :)