The longest Interstate I haven't been on is I-10. I-90 is probably the Interstate I've been on most often, and I've been on I-80 many times as well (including the portion that's 80/90). I've only been on I-40 a couple times, but I have been on it, so I-10 it is.
Growing up I generally thought of the Midwest as the states with "traditional" Big Ten universities, i.e., Ohio, Michigan, Indiana, Wisconsin, Illinois, Minnesota, and Iowa. I'm willing to accept a larger Midwest with Great Lakes and Great Plains subregions though it gets tricky once you start considering whether all of this or that state is truly Midwestern, or if it's just part.