The WebServicesFaq lists various implementations, and points to many more. Instead of starting by looking at implementation libraries, I'd advise to read up on concepts such as SOAP and REST and their Java APIs SAAJ, JAX-WS and JAX-RS.
By the way, when comparing implementations (or even technologies), there rarely is a single "best" in a general sense. It always depends on the circumstances.
No one can make you feel inferior without your consent - Eleanor Roosevelt. tiny ad: