Back in the day of the original GSC it was Whitney. Her Miltank was a nightmare and while it's still a challenge, the one that gave me the most problems in my Soul Silver game is Clair's Kingdra. The thing scored several critical hits on me with Dragon Pulse alone. It took down two of my best pokemon with Critical hits. I detest you, Kingdra. Morty is also another one that was much harder in the remake. Mainly because his Gengar didn't try to spam Hypnosis the entire fight and went right for the Shadow Ball. So Whitney, Morty, and Clair for me. Blasted Kingdra.
Parena or Serecham both equal Serena+Pancham! Sereevee or Serevee equals Serena+Eevee!