Vegeta called Goku the strongest upon defeat Buu meaning he's the undisputed #1 (undisputed, this means that it's not debatable, Gohan or Gotenks being strong invalidates it and makes it an illogical statement. The writer wouldn't state something to lie to his reader).
So upon defeating Kid Buu Goku BECAME the undisputed #1.
a) Vegeta's rivalry is taken out of the equation at the time when he called Goku the #1 between himself & Goku, as he and Kid Buu were fighting.
b) Upon Kid Buu's defeat Goku becomes the undisputed #1.
So clearly they are stating different things and incorporating every possible fighter who could potentially surpass, including Gotenks & Gohan -with Goku being the #1 in the end.
Rivalry has nothing to do with it.