First Gym Leaders for me are always lolgg worthy.
Foremost, you've got Brock, who trains in Rock-types. Only they're also Ground-types, which means, haha, he's 4x weak to two out of four starters. Way to go, Brock. Hold on while I hit you with Vine Whip and Bubble and sit back to laugh about the fact that my Pokemon can OHKO you while they're still underleveled. It didn't help that they gave Charmander Metal Claw in FrLg, as if to let Brock get completely pwned by all three starters, and in RBY, Bide totally made even Onix rape bait. ("Oh, you're using Bide? K, I'll just sit here and Growl you to death while you work up your ineffective move. And then I'll Scratch the crap out of you. And then we'll repeat this cycle because the CPU in this game is asking for it!")
In GSC, yeah, you've got Falkner and his WTFlvl9?! Pidgey. I didn't even intend on overleveling for this Gym. But, uh, I did. And then I beat the crap out of his birds. I'll have to agree about the TM, though. WTF, Mud-Slap?
RSE, return of the Rock-types-on-first. Roxanne was slightly harder than Brock (if only because her strongest Pokemon only had a 2x weakness), but come on. Two of the starters have a type advantage still, and the other one evolved into something that could literally kick rock tail. On top of that, let's face it. Rock Tomb? It missed so frequently I had to lol.
DP, same thing with Roark, essentially.
In other words, I'll have to vote for all of the first Gym Leaders. Can't decide on any one of them, personally. I plowed through all of them with just my starters or the first Pokemon I caught (in Falkner's case).