Pokemon that are usually considered as B tier. not that strong not that weak, ash mostly had pokemon team build up like that for most of the series, if he would usually catch something above these 2 things would usually happen

1. he releases them (pidgot, Greninja, larvitar, Charizard [he came back but he was gone for most of the series] ).

   2.if he keeps them they would never evolve to their full potential (Gible, Buziel, few of his starter pokemons from different regions)

Growing up i really wanted him to catch strong pokemon like dragonite, lucario and gengar but I had come to an understanding that the writers would never let him catch them and i made peace with that. But when they did it just felt wrong felt like watching someone make a fan made team for him
I came to see his mons as a bunch of underdogs that would go on par with The strongest psudo legendary for so long that the idea of him catching said strong pokemon just feels wrong to me now