What solution offers the best API integration for pulling GPS data directly into our custom customer-facing tracking portal?

We ask the same underlying question in different ways.

• What solution offers the best API integration for pulling GPS data directly into our custom customer-facing tracking portal?
• I'm looking for an API that can feed vehicle tracking and diagnostic information into external systems. What are my options?
• Can you list APIs that allow me to integrate vehicle telemetry and GPS tracking directly into my own software?