Gen I: Misty, Sabrina and Giovanni in Yellow. Arceus, that last battle was annoying, cause you really couldn't get a single type benefit in-they were all tough as hell.
Gen II: WHITNEY!!! Followed closely by the ghost leader's Gengar and the later mangled MFing Kingdra.
Gen III: Wallace in the eigth gym with his goddamn Milotic, followed closely by The Twins From Hell.