MIT's most populus major: Electrical Engineering and Computer Science. Nearly 1 out of every 3 MIT students declare Course 6. On 4e the percentage is about 3 times that (or so it seems).
Within Course 6, students choose among 6-1 (Electrical Engineering), 6-3 (Computer Science), or 6-2 (a little of both, for the
indecisive "well-rounded" types). Slugfest is currently heavy on 6-2s, with quite a few 6-1s as well as 6-1s who are pretending to be other majors (such as 2 and 8). They'll come around.