Whitney and her Miltank always pop into my mind when I think about my hardest gym battles. It's actually not as bad(but still a pain) in G/S/C because there are loop holes to get around like getting a female rock, steel, or ghost type. It's the one in the remakes I curse straight to hell with that hacked Stomp, holding a Lum Berry that completely ruined the hell I went through from all that flinching to finally poison it with my Weepinbell, and found out later from my little cousin that it has an ability that lets it hit ghost types. I was lucky I had a female Onix so it was slightly easier, but the thought what would happen if I didn't *shudders* Still, I can't complain too much because Whitney does warn you before you battle her.
Oh, and the fact that Miltank is almost as fast as a Starmie or Gengar has always confused the hell out of me.