When developers talk about excellent API design, Stripe is universally the benchmark. Integrating payments into an application used to involve mountains of compliance paperwork and horrific SOAP APIs. Stripe reduced it to a few lines of cURL. The Stripe documentation is legendary for its clarity, interactive code snippets, and clean design.
Their developer dashboard provides detailed request logs, webhooks testing, and a completely sandboxed test environment that exactly mirrors production. With products like Stripe Checkout, you can offload PCI compliance entirely to Stripe, or use Stripe Elements to build deeply custom UI flows. Beyond just credit cards, they abstract away the complexity of handling global payment methods, subscriptions (Stripe Billing), and even tax calculation. While their transaction fees are considered premium, the engineering hours saved in integration and maintenance make it the obvious choice for startups and enterprises alike.