The final battle of the Buu Saga is actually between two of the weakest characters/versions of characters that show up in that saga.
Vegito, Super Buu (any absorbtions, or none), Buff Buu, Mystic Gohan, Gotenks, all of them are stronger than Goku and Kid Buu. Possibly Majin Buu, as well (the version that comes out of the egg) - We never truly see his full power as even against Goku, he was playing around.
Only versions of major fighters in the Buu Saga that are weaker are Goku and vegeta as SSJ2 and probably Evil Buu and Mr Buu (the split forms of Majin Buu)
In fact, Goku's strength doesn't change one bit in the Buu Saga. When he uses the Genki Dama on Kid Buu, he's basically the exact same strength he was when he came to the earth for the tenkaichi budokai. Which makes the whole saga rather pointless as Goku literally had no progression.