The Gym Leaders are an interesting case, because their gimmick was to be basically just that you have the strongest species in a given type, or at least what is considered such, and in some cases the only ones of that type, and then give them to one trainer who is to use them. In this sense, they're not actually that clearly divided from the player, and in that sense in asking which of them the player likes you might also be covertly talking about them, as in terms of style that is quite similar. Gym Leaders are generally determined in nature by this overall purpose.
You suppose that Erika, other than having her type's weaknesses helpfully pointed out in her name, as well as her anime incarnation described in its backwards form, generally has the most passive Pokémon, and relies quite a lot on luck without generally getting a chance to show much of what they have due to weaknesses such as Charizard, the early bug-types, poison, and so on. Blaine isn't that interesting, although the Seafoam Islands might be of interest due to being near them and also containing primarily Pokémon they would have issues with - while Cerulean, for instance, would mostly be known for its nearby Abra - while Giovanni's Pokémon are a bit of a gimmick (and Gen. I's trend of ending things on recurring characters is slightly annoying, although these characters were worse from Gen. III onwards and Gen. II with Lance, mostly, although the game 'ended' with Red), Lt. Surge is mostly just about type match-ups, and Sabrina is generally either a case where an early bug Pokémon might be of some use, or more conventionally can just be burnt through by other Pokémon, such as fire-types. Ghosts not being super-effective there is a bit of an annoyance - less so against Mewtwo - but perhaps they were too busy laughing. I'd suppose that what you think of as their own capabilities, rather than their relative niche compared to the other Gym Leaders, might just reflect the player's play-style. The Gym Leaders get quite plain towards the mid-game, though. You might say Koga, ultimately, who somewhat embodies that progression, and deserves some credit for doing that, on their own account.
