You really dont need any specific field, I was surprised at the vast diversity at Kellogg's admitted students weekend. I think where you stand in comparison to the rest of the people in your field is just as important as what field it is. Yes some fields are more difficult to get into but having the brand name company in that field is just as important, as is your success. If you are a consultant for BCG its going to look a lot better than being one for a no name company...if you are an IT person for Google or Microsoft its going to be more impressive than some random little software company. If you have been promoted to a position far quicker than the average applicant thats going to look really good.
In a way being an IB or MC is tough, since you need to come from those big names and have solid career progress...they see lots of those applicants so they can pick the cream of the crop. Where as some fields have far less competition and much more diverse employers...so its harder to compare people.