I like the ending of bonk III. Hudson knew it was over. And it was sort of sad with the music and Hudson's message to the "Bonk kids." That was the best part for me, which isn't saying much. Although the music in the 7th stage/Drool battle is very good.
Overall, though, as much as Bonk holds sentimental value to me (see avatar) especially the fist two games, there is simply no challenge in each Bonk game. They are too easy, which I think is the biggest problem with the series.