Which Ivy League school is the most difficult?

You'd actually be surprised, but ivy league schools tend to be easier than state universities because of rampant grade inflation which has been a major topic of debate for at least four decades.  A student who gets a "B" at a Cal State, for example, is said to do almost twice as much work and is graded much more harshly than a Harvard "A" student.  I have done a few certification classes at ivy league schools (Harvard for Data Analytics and Cornell for Business Statistics) and they have all been a cake walk.

Difficulty is not what separates the ivy leagues from other schools - it is their endowment size, their resources, and access to the most up to date technology.  

There is no way anyone can answer your question sensibly.  The person who got a BS in Physics from Harvard did not get a BS in Physics from Yale, nor a BA in English from Harvard.  And even if one person did get all three, how would you like them to define "most difficult"?

This is going to be really difficult to assess. As an example, I went to Cornell for undergrad (and have advanced degrees elsewhere), but I only had one undergrad experience - I don't have a point of reference at other Ivy league universities to make a meaningful comparison.

For what it's worth. I would say that Cornell was very challenging. I worked hard and studied - a lot. It was tough to keep up with all the work. I did my master's degrees at State Universities, and my perception is that the undergrads don't work as hard there as I had to at Cornell, but I can't speak for other Ivies. 

I would say it's also very variable by major. 

all of the Ivys have multiple colleges and curricula .. and no one has much experience with more than two [usually].  You'd do better to focus your attention on the job placement prospects of each of the schools you're considering in your specific field.