In RSE, it had to be... gaaaah.... Norman & Winona, although over time Winona got a little easier... until you met that Altaria of hers. What a beast! Alty killed my Mantectric, but I finally silence her with my Swampert and my Vibrava. (Why Vibrava, I don't know, since Vibi coulda gotten killed by Alty. Meh. I still beat her >D)
With FRLG, it had to be... um... well, they were pretty easy, but my least favourite was Sabrina. Not that she was hard, just that she was hard to get to xD (gah, like Flannery in RSE, but I kicked her butt >D) Sabrina's pretty easy, though.