It's difficult to say based on acceptance statistics because there is a "self-selection" that takes place at some schools. For instance, a lot of people think of MIT as a boring, quant-based program, so people who do end up applying might actually be people who are looking for a rigorous program to challenge them, i.e. stronger candidates. This is just conjecture though.
Based off of admittance numbers, I think Chicago may have the highest percentage of accepted applicants, although similar to MIT, I would guess there is a self-selection process that causes stronger candidates to apply there in the first place.
Harvard and Stanford appear very selective from their stats, but again if you think about the people who apply to these schools, you have plenty of strong candidates as you would at Chicago and MIT, but then you also have weaker candidates like me who say, "What the hel, let's swing for the fences and throw an application in to Harvard and see what happens!" So people like me are probably inflating the perceived selectivity of some ultra-elites (and I mean "inflating" in the sense that "rational players" wouldn't just "swing for the fences," they'd pick a realistic reach school).
Harvard and Kellogg are also thought of as more "fun" schools to attend. So this probably attracts more candidates (weak and strong) across the board.
So let's say you're a candidate whose "strength" or "quality" is "X," you're probably going to be going up against a PROPORTIONATELY higher number of candidates that are greater than X if you apply to MIT or Chicago, than if you apply to Harvard or Kellogg.
At the end of the day, if you're set on going to an ultra-elite, I'd say your best chances are Columbia or Chicago if you have a strong quant background and Kellogg if your profile is stronger based on interesting life/work experiences.