woot! my vote brings houndoom into the lead :)
For those who believe that Houndoom is good as it is, look at Electabuzz, Rhydon and Dusclops. They all have base stat totals similar to Houndooms, and before the evolutions from DPP, they were also UU/OU/BL. All three evolutions are not going to overpower the metagame like Skymin or Scizor, but they can very well make you pay when you're unprepared. I believe this is the type role that we would want our first EVO to be.
Electabuzz, Rhydon, and Dusclops can be effective in OU but not effective enough to make them pay for being unprepared. Houndoom CAN be effective, but it just needs a little more in order to make an impact on the metagame.
Electivire is famous for hitting 13 of 17 types for SE damage, alongside great attacking stats and a means to increase his good speed via Motor Drive. Rhyperior is a force to be reckoned with, especially with Sandstorm and specially defensive EVs and natures. It can take hits and hit you back with its amazing attack stat. Dusknoir replaces Dusclops as a rapid spin blocker, and the increased stats definitely help.
All of these evolutions aren't "necessary" for these Pokemon to be great, but it definitely helps.