For me its Whitney and her freaking Miltank. I don't know if its just me but whenever I fought her both in Gold and Soul Silver Miltank did nothing but Stomp which caused flinching more then half the time and Attract which took care of anytime Stomp didn't work. Top this off with how if Miltank got down to 1/3 of its health or less she uses a super potion makes me want to curb stomp her until it stops being funny. Which it wouldn't.