Out of these I'd definitely say Whitney's miltank. I remember having pidgeotto and quilava and she ko'ed em both easily. Blues zam was fast but not too much of a problem. I don't remember metagross or milotic being tough. They're both slow and defensive, it just takes a few hits. Don't know why hydre was such a problem for most BW players. I had a 4-nonlegendary or hacked-in-any-way-Pokemon team at lvl45-50 and smashed through ghestis like nothing. Cynthia's garchomp was always a problem though. BW garchomp ko'ed 3 of my Pokemon in a row.