In modern architectures, use all three: OPC UA for real-time control loops, MQTT for cloud telemetry, and REST for ad-hoc queries and UI.
curl -u "username:password" "http://192.168.1.100/WinCC/REST/Tags/MyTag" wincc rest api
The is a modern web service interface introduced in SIMATIC WinCC Professional (TIA Portal) and WinCC Unified (the latest generation of Siemens’ HMI/SCADA system). It allows external clients (e.g., web dashboards, MES, ERP, third-party applications, or custom scripts) to interact with WinCC runtime data over standard HTTP/HTTPS protocols. In modern architectures, use all three: OPC UA