That's a dub line.
And the anime did incorrectly make him out to be the strongest. I.e. It's non-canon. Let me fish out some manga quotes.
Rou Kaioshin's Comments on Power Up
"If you can swing that sword around already, you'll really be the strongest under the heavens by the time I'm done with you!" (in regards to power up ritual)
"Feh. Transforming isn't everything. That super-whatever is just showbiz." (in regards to latent ability and super saiya-jin)
Goku's Responses to Chou Gohan's Power
"Latent power" is hidden power he had inside...right? (continued..) How can that be...? ...How much power did Gohan have hiding...? (Goku's response to Gohan's incomplete power up)
"!! (continued...) You're right...it is amazing...it's insane...! I don't believe it! You haven't changed on the outside...you're not even super saiyan, but...I didn't know this was even possible..." (Goku's response to Gohan's complete power up)
Goku and Vegeta's Comments on Their Abilities in Contrast to Shin Buu
"Wait! He's still stronger than either of us! He'll kill us if we go out like this!! (continued...) You just had to break the potara!" (Goku in regards to Shin Buu's power)
"Now this looks like...it could be trouble...!" (Vegeta on Shin Buu's appearance within his own body)
"I told you to keep the potara on!! This would've been easy if we could combine!! (Goku, after Shin Buu's entry)
Shin Buu in Regards to his Ability in Contrast to Goku and Vegeta
"!...So...you can't combine anymore...hee hee hee...that's nice." (on Goku and Vegeta's inability to fuse)
"You can't do it. You're gonna die." (In response to Goku's statement "...So we have to beat you first...?")
Comments on the Ability of Chibi Buu
"Heh...Heh Heh Heh...look at our little friend!" (Vegeta, in response to Shin Buu's degression into Chibi Buu)
"Yeah! We can take him!" (Goku, in response to Vegeta's statement)
"Buu!! You can blow up this planet if you want!!! But do it AFTER you've fought us!!! (continued...) Heh heh heh...that's better..." (Vegeta, in response to Chibi Buu's initial attempt at destroying the earth)
"You'll destroy the earth!!! Don't you want to fight us?!" (Goku, in response to Chibi Buu's Chikyuu Hokai no Ichigeki)
"...No thanks. We'll do without 'em." (Goku, turning down the potara earings)
"You sure? You might not get your turn." (Goku, in response to Vegeta's remark "I want to see this super saiyan 3 up close...")
"What?!!! He's asleep...?!!! Who does he think I am...?!" (Goku....in response to Chibi Buu sleeping, obviously...
)
"Don't mind me. Finish him off! Build up your chi with Super Saiyan 3, and you should be able to obliterate him!" (Vegeta, on Goku's power as a Super Saiya-jin 3)
"...So Skinny Buu spit him out." (Rou Kaioshin, directly equating Chibi Buu to Skinny Buu)
According to the manga atleast, Gohan Buu>>>Gotenks Buu>>>Buff Buu>>>Piccolo Buu>/=/<Shinn Buu>>>Fat Buu>>>Chibi Buu>/=/<Skinny Buu>>>Good Buu
Toei makes a lot of dreadful mistakes, and FUNI makes five times more.
As I've said,
canon discussion please, meaning the manga, not the anime. Those comments on Chibi Buu's power were never made in Toriyama's work.