Welcome to the club, Symbonite, hope you'll have fun.
Ignoring Koga & Wallace, which Gym Leader(s) is worthy to be an Elite Four or Champion? Explain your reasoning.
Uhm, I'd say either Sabrina or Whitney. Both gym leaders were tough opponents, and had well trained Pokémon, and good strategies as far as the AI brain goes. While Sabrina relies on her complete team to finish off the player, with as an addition a few of the Pokémon are stronger than others, her battle can wage on for a while if you're not prepared, as well as, if I remember correctly, her Hypnosis attacks are a real pain in the butt.
Whitney on the other hand is mainly tough because of her Miltank, though Clefairy should also be aknowledged. If you used male Pokémon against her Clefairy, the attract would just be a constant annoyance untill it was finally finished off, and Double Slap can really damage you by that time. Though after that pain has been taken care of, she sends out her ever so annoying Miltank to dominate you with a combination of Rollout and Milk Drink, which has left me raging many times ;) Only thing that she would need to be qualified for the E4 team would be more Pokémon, as a team of 2 Pokémon can't really cut it for that position.
So yeah, both have unique battle styles, and both are equally effective, so yeah.