I personally found Brock's Croagunk to be really boring. I know it was meant to be a comedic Pokémon in Brock's party, but after seeing it the first few times, I just grew weary of it being around. The only time I really had any likeness towards it was when it created a rivalry with Saturn's Toxicroak.
Happiny would probably my most favored Pokémon of his though. To me, everything Happiny did, especially when it showed off its super strength, came off as pretty comedic in the show. It really had a good nature for Brock's character and more so showed off his breeding intellect. I was a little disappointed when it evolved into Chansey though, because I figured its personality would alter into the generic Chansey we all know. Luckily, it evolved at the end of the series and not any time earlier than that.
Close second would be between his Marshtomp and Ludicolo.