I think it's the decision part - to join which school.
wish i could say that...
this heavily varies - I had a huge battle with the gmat unlike most others. Really, i think writing and thinking of essay is supposed to be the most challenging on average. The entire process can be extremely stressful, unless you're confident, killing it, and getting admits all over the place.
1. stress (more personal b/c i am still wondering what the hell im going to do)
2. essays (standard #1 in my opinon)
3. gmat (all thought this was harder for me personally)
the rest is just easy in my opinion....it takes time and hard work, but that = results for these things in my opinion, unlike things like the gmat and essays.