Brock is arguably the best companion Ash Ketchum had on his Pokemon journey, and he had some incredibly strong partners.
Brock is best known for his way with the ladies. He was always trying to get into the hearts of Officer Jenny, Nurse Joy, or any other poor girl he came across. That often distracted him from his skill as a Pokemon trainer.
He was the Pewter Gym Leader after all. Brock and his Onix, which eventually evolved into a Steelix, were a force to be reckoned with. Some other Pokemon in his care though, were probably better off elsewhere.
Note: This article is subjective and reflects the opinion of the writer.
Top 3 Pokemon Brock shouldn't have trained in the anime
#3 - Mudkip
Brock's Mudkip was just as stubborn and flirty as its trainer. This is especially true for when it evolved into Marshtomp. That saw it become distracted in battle a few times due to the move Attract.
It wasn't a terrible battling Pokemon, but it certainly can't be considered one of the greatest. Brock never took it to its full potential as a Swampert. As he joined Ash on the Sinnoh journey, Brock simply left Marshtomp in Pewter City.
#2 - Vulpix
Vulpix was a Pokemon given to Brock, rather than one he caught. Much like Marshtomp, Vulpix never truly reached the top tier of its strength. It was also left behind, albeit this time it was given back.
Vulpix was an average battler, but didn't have enough screentime to show off what it could do. Brock returned Vulpix to Suzie, indicating he truly believed he was just watching over it for a short while. That made it feel completely pointless.
#1 - Geodude
Geodude is one of Brock's most notorious Pokemon, in the games and the anime series. Geodude and Brock are one of the longest-known partnerships in the series. While Geodude had its fair share of important moments, it never evolved. Like many of his Pokemon, Brock left Geodude behind at one point.
Once he returned, it was still a plain old Geodude. Unlike Onix, Geodude was never seen as Brock's strongest Pokemon. At this point, it seems like Geodude would have been better off with a different trainer - one who would evolve it and dominate opponents in battle with it.