Another choice of subcluster that comes up relatively frequently is Harvard, Penn, and Stanford (usually rendered as "HSW" or something similar). I have yet to discover why this particularl subcluster seems to hold it "true believers" in such a state of awe.
Remember, if we include three schools it means we have necessarily excluced the others of the seven. Accordingly, Northwestern/Kellogg, Columbia, Chicago, and MIT are all available to unseat any of these three schools.
Stanford, an outstanding school by any measure, is still probably the easiest to unseat (as described in the post above Kellogg can match it in finance and strategy, Ch/Co can handily beat Stanford in finance and hold their own in the softer skills).
Penn/Wharton is a bit tougher since it has so many strengths. However, Northwestern would provide extremely tough competion on the softer skills and Ch/Co are at least as strong in finance. Either way, it is probably not far enough away from the others to justify grouping it separately.
Harvard is a tough case. First, no school can come close to matching its CEO production. Further, the gigantic FT class size means that one has a large number of network opportunities. A perhaps even greater problem in unseating Harvard is that Harvard reveals so little information about its graduates. Thus, it is hard to perform much of any comparative analysis between Harvard and any of its competitors.