In the old days (Gen II), Bugsy's Scyther, Whitney's Miltank, Morty's Gengar and Jasmine's Steelix were all pretty challenging. Whitney more so than the others because I simply could never figure out a good strategy to deal with Miltank.
When I played HG, though, I didn't have much of a problem with any of them. Golbat, Heracross, Trapinch, and Rapidash assured my victory at each gym, respectively. Although Morty's Gengar did have me on the ropes, the poor success rate of Hypnosis gave me the only opening I needed. Guess I got a bit lucky on that one.
Now, when I get my Japanese SS I might be singing a different tune. The team I'm planning on taking all the way to Mt. Silver is Ekans, Psyduck, Vulpix, and Meowth. Hopefully with excessive use of the disruptive moves Disable, Glare, Screech, Hypnosis, Confuse ray, Will-o-wisp, and Flash, I should be good to go.