Which attribute provides a unique value to view a transaction end-to-end?

The W3C trace ID uniquely identifies a distributed trace across all services and components involved in a transaction.

It enables:

End-to-end visibility of a request

Correlation of spans across multiple services

Full trace reconstruction in distributed environments

Other attributes are not unique identifiers:

Span kind describes the role of a span

Process ID identifies a process, not a transaction

Service type categorizes services

Only the W3C trace ID provides a globally unique identifier for a full transaction.