It's usually between Whitney and Clair, I find. In GSC, Whitney was VERY difficult to beat because of her Miltank, but in HGSS something about her just seemed so much more managable. Clair on the other hand was harder than ever to beat; in each playthrough of HeartGold I find that I only ever beat her with the skin of my teeth, and I never had anything that could handle Kingdra too well- I the last time I fought her I basically had to wait til it could only use Hyper Beam. :/