Gen I: Sabrina. Misty was challenging the first time I played because I picked Squirtle and had to rely on my Paras using Bullet Seed.
Gen II: Whitney's Miltank. It kept infatuating my pokemon -_-
Gen III: Tate and Lisa. Norman was hard when I picked Mudkip as my starter.
Gen IV: None of hem were that hard for me, but Fantina in Platnium gave me trouble.
Gen V: Elesa and her god damn Volt Switch Emonga.