I'd have to say Dragon Dance. Mixmence was only effective because of the threat DDMence posed. If Salamence only had one viable set, it would still be OU. Without Dragon Dance, Dragonite has to rely on MixNite or being a Cleric, Gyarados is BL, Altaria is useless save for the uncommon Band, TTar becomes easy to predict (as no-one runs BandTar anyway) and Whiscash is even worse. Other pokemon that get it are Feraligatr, Crawdaunt, Tropius, and Charizard, most of which are unaffected, either due to being awful anyway or having better/other options. Rayquaza is forced to use Swords Dance or Mixquaza (both very good sets anyway), and Kingdra doesn't need to run DDance, there is a nice mixed set as well.
SR is good but Articuno, Charizard and the like weren't that good anyway; the only pokemon seriously hampered by it is Yanmega.
I would argue that the key move on Breloom isn't Spore, but Substitute. It can 6-0 without putting anything to sleep, it just gives it a slightly easier time. The main attraction to Breloom is unhampered Focus Punch, and what causes that? Sub.