Gen 1 - Sabrina due to Alakazam
Gen 2 - Clair due to Kingdra
Gen 3 - Tate & Liza in Emerald due to Claydol constantly spamming Earthquake and Xatu spamming Psychic for starters.
Gen 4 - Maylene because her Lucario was quite the pain in the ass to take down back then.
Gen 5 - Lenora without any doubt in my mind, I really did not like being OHKO'ed by that damn Herdier's Take Down whenever I was so close to killing that retarded little flea bag, even my Pignite had trouble with that thing at the time but thank god I scored a critical hit on one of the Arm Thrusts which took it out only to have the Watchog but that wasn't as tough as the Herdier IMO even with Hypnosis and Crunch.