It isn't, needless to say, the undergrads who study Political Science. Here, courtesy of the Department of Philosophy at Indiana University-Purdue University Indianapolis, is a ranking by the average LSAT score of students in each major that had at least 400 students taking the LSAT (the score is in parentheses):
1. Physics/Math (157.6)
2. Philosophy/Religion (156.0)
3. Economics (155.3)
4. International Relations (155.1)
5. Chemistry (154.5)
6. Government/Service (154.4)
7. Anthropology/Geography (154.1)
8. History (154.0)
9. English (153.7)
10. Biology (153.6)
Finance majors came in 12th (with an average score of 152.5) and Political Science majors were 18th (151.6 was their average score). The bottom seven majors, in terms of LSAT scores: Management, Business Administration, Health Profession, Education, Prelaw (that's a major?), and Criminology.
I'm sure the Philosophy result is dragged down by the fact that it is lumped with "Religion," which as an undergraduate major is much closer to history and sociology than to philosophy.
The impressive results for Physics, Math, Philosophy, and Economics must, one suspects, be credited in part to self-selection, but some surely reflects the intellectual rigor and demands of these courses of study. I am pleased to note that Philosophy majors, even though dragged down by Religion, outperform Economics majors. This, of course, corresponds exactly to the natural intellectual hierarchy evident throughout the legal academy!