Bronzong is my least favorite because every single stupid Elite Four member G4 and beyond gets it sooner or later. A Lv. 100 Giratina can only 2KO it with shadow ball if the opponent's Bronzong is Lv. 55 or higher. There must be a reason why it's at the top of the list on 'Steel types....must....DIE's' home page.
And I'm surprised that this forum hasn't been updated to include B/W Pokémon. The PKMN in B/W that I hate the most are Ferrothorn, Bisharp and, Klinklang.
Bisharp for the obvious reasons: It has access to Guillotine and if it survives, it can land a OHKO on an Emboar using Aerial Ace.
Klinklang because... You know what? Take a Thundurus to the Battle Subway in Nimbasa and see how well you fare against this guy at the single trains.
And now for Ferrothorn... This one is ->|<- that close to being moved to number one on my Hated Steels list. I entered a tournament and I sent out my Arceus to face him using Flamethrower, but the guy switched out and sent out an Infernape, so I proceed to use Fly (hesitantly because Infernape is my 2nd favorite PKMN) and he switches on the second turn back to Ferrothorn. Iron Barbs brought Arceus' HP down to red. This pattern repeated 2 more times and Ferrothorn killed me and I lost! No blame goes to Infernape because all he did was stand around for about 15 seconds each time.