To be fair, in SW, the Force ghosts never do anything particularly powerful. (Even in the EU, the most powerful Force ghosts, like Exar Kun, usually must act through agents.) If Obi-Wan's statement is anything more than rhetoric (and it's never safe to just rule out 'it's rhetorical/symbolic'), it's a reference to Luke, who at least in the EU is basically the most powerful Force-user ever.

Further reading: Wookieepedia