Lance, without a doubt. Although he isn't an Elite Four in Johto--Champion, rather--he plays a major role there, helping to chase Team Rocket off the region. That, if you ask me, is a major task (considering how big they are), and he was quite successful at it, too. He did get assistance from Gold, but Lance still did help nonetheless, and that's all it matters. Plus, he's also quite tough. When I battled against him for first time in Pokémon Blue, taking this guy down was literally impossible. I did manage to beat him at some point, but it was still tough.
One of my other favorites would be Drake. One, huge reason why I'm fond of this guy is his backstory relating to the abandoned ship in RSE. Personally, I find it to be quite intriguing. Though, if it were more elaborated (and more integrated into their main plot), it would've been a lot better. Also, just like Lance, this person is extremely difficult to beat as well. I've had to rely on countless amount of recovery items to successfully get it done.
As for others, they'd probably have to be Bruno, Will, Koga, and Sydney.