To have a pokemon based around momentum, it has to be able to stop the momentum of commonly used pokemon.
Arguably, the most common attacking types in OU as of now are
Dragon, Fighting, Ground and Water. All of which have rather powerful STABS, or, in Water's case, have Double STAB due to the weather.
Obviously, no single type resists all of these. In fact, no duel type can resist all of these types [Only Steel Resists Dragon, and it's weak to Fighting. Going Steel/Ghost will leave us weak to Water, even with Levitate]
In addittion, I feel that if we go, as many people are suggesting, with Steel, we will just end up with Nattorei/Scizor. Let's face it people,
when it comes to steel types and momentum, Nattorei and Scizor have it covered.
Nattorei can come in on anything that's not fighting or Fire, and threaten with 120 BP STABs off of a more-than acceptable attack stat. That screams momentum to me. Sure, he cannot U-Turn or anything, but he does what the Steel-type does best, stopping mometum. When it comes to gaining mometum, Scizor has Steels covered in that department, with his typing, bulk, threatening Bullet Punches, and U-Turn.
Personally, I feel we should go with the
Flying type.
Why is this?
#1: The Flying ype provides an immunity to Ground, resistance to Fighting, and neutrality to Water and Dragon.
#2: The Flying type has some decent moves now, and generally is wide open in terms of movepool. Tailwind and U-Turn are also good moves to swing mometum.
#3: One of the biggest throwns in the Flying type's side, Stealth Rock, is used a lot less this generation. Between the loss of Spinblocking in Rotom-A, Mischevious Heart, Team Preveiw, and Magic Mirror, as well as things that you can't give that free turn to set up SR on, such as Blaziken or Renkulus, SR is no longer what I would consider a Standard Battleing condition [Indeed, I see more Spikes than SR... probobly due to the fact that they hit Nattorei harder]
Flying can be nice too. It's an underused type due to the lack of good Pokemon / attacks, but it can be interesting to see how a Flying Pokemon not-named Gliscor can change the metagame.
Zapdos in Gen 1 -> 4.
Dragonite in Gen 5
The Genies
Skarmory
All would like a few words with you.