OP, use "among" to refer to more than 2 objects. examples: among the 3 of us. Among Harvard, Princeton and Yale.
use "between" to refer to two objects. example, between the you and me. between Stanford and Berkeley.
but this is a common mistake in english grammar. hehehe
anyway, of all the schools that admitted you, NYU and DePaul are the strongest. but that is just my personal opinion.
If Northwestern will accept you, NU will be the strongest school in your list, based on reputation.