If we're talking historic old money/blue book WASPs, you can find your answer in this article from 1963, "Colleges of America's Upper Class"
https://www.unz.com/print/SaturdayRev-1963nov16-00068/
It's Yale by a lot. Followed by Harvard, then Princeton. Then there's a sharp drop to everything else. For women, it's 7 sisters + junior colleges, more evenly mixed.
Summarizing here:
Colleges most commonly attended by the 1963 college-aged cohort:
Yale (171) > Harvard (123) > Princeton (76) >
Pennsylvania (44)> Trinity > Middlebury > Virginia> Stanford> Williams> Hobart/ North Carolina (tied)> Boston U.> Dartmouth
For women college students, also decreasing order/10+ students listed):
Smith> Vassar> Radcliffe> Wellesley> Wheaton> Bryn Mawr> Sarah Lawrence> 3 junior colleges (Bradford, Bennett, Briarcliff)> Hollins> Connecticut College> Mt. Holyoke> Wells
And for the alma maters of the men listed in the blue book:
Again, Yale (2234), Harvard close behind (1746), Princeton 3rd (1422)
Then there's a sharp drop:
Williams (325), and it goes down from there:
Columbia> Virginia> Cornell> Dartmouth> Amherst> M.I.T.> Trinity> Pennsylvania> Brown> Annapolis> West Point > N.Y.U.> California (Berkeley)> Georgetown> Colgate> North Carolina> Hamilton> Wesleyan