If we are talking about all 4 Black Ops combined (PS3,360,Wii,PC) then obviously Black Ops. But that wouldn't be an impressive achivement as having more platforms to sell to means it has x3 more audience to sell to.
If however were talking about on single platforms then it is no competition, Reach will destroy Black Op sales.
The last Halo game which was released last November ODST has already sold over 5m units. To put that into some kind of perspective if ODST was a PS3 game and had those numbers it would be joint 3rd biggest selling PS3 game. Selling nearly 25% more then W@W. On 360 it is the 8th biggest selling game.
Reach is being seen as the next/last proper installment of the series. Of which the last proper one sold 11m units making it the biggest selling game of the HD consoles generation. If Reach taps into those people's interest then it has a chance to exceed those figures.
So on single platforms = Reach. All of the platforms combined = Black Ops.
