In your opinion, do you think it is ECs, grades, etc.?
With the mega high number of applications, maybe luck.
The single biggest factor is that there is no single biggest factor.
A great hook to get into one of the favored pools would be the single biggest factor for anyone who already has the “goods”, that is the midstream test scores/grades for a given college. Test scores are a big deal once a student has a certain threshoold in grades. So it’s difficult to pick one factor out all of them without something else in conjunction.
The single biggest factor, in my opinion, is the set of needs and goals colleges use as a guide to create their freshman class. Unfortunately, we have no way of knowing what those are.
Grades and transcript. Always has, always will be. Colleges are learning institutions first – not summer hire programs or summer camps that want “team leaders”.
Only in the rarefied environment that most CC users exist do ECs come into play since they’re vying for the top 5% of US colleges which need something to differentiate 1000s of high achieving scholars. Hooks can’t be invented; either you have one or you don’t.