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.
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