What is most suitable for connecting SAP Cloud Platform to ABAP backend?

There is no best tool for connecting SAP Cloud Platform to SAP ABAP backend since it depends on the scenario.

For point-to-point connectivity using BAPI, RFC, and OData services in on-premise systems can be exposed to outside via SAP Cloud Connector. When you need advanced functionality like n-to-n connectivity using an integration hub, SAP Cloud Platform is the best tool.