Typically, the answer you get when you ask this question is "political science" or "literature" or some other similarly non-marketable degree.  These degrees may very well help you understand the basics of the U.S. governmental/ legal system and/or help you develop your reading comprehension skills.  They may also be useful in terms of providing you with the GPA you need to get accepted into law school.

However, four years of college which serves no real purpose other than law school prep, seems like a pretty big waste of precious tuition dollars.  Worse, what happens if you decide you don't want to go to law school after all?  Or even worse, what happens if you don't get into law school (although, for reasons I will explain in subsequent posts, this is actually pretty unlikely)?

As a result, I recommend that you think about a more marketable degree, supplemented by concentrations in law-school prep activities.  The biggest payback law school prep activities you can engage in are, generally speaking, reading comprehension, effective writing and analytical reasoning.  The first two are fairly easy to schedule academically, and in fact, most undergraduate courses (with the exception of the hard sciences and mathematics) have components of both.  Analytical reasoning is a bit more tricky, but the good news is that there is a ton of literature dedicated to exactly that.

If you earn a marketable undergraduate degree, but keep your eye on your ultimate goal of a law degree, you will allow yourself more career flexibility in the long term.  More importantly, certain degrees, such as accounting and business, will hugely complement a law degree.  This, in turn, will make you that much more marketable as a lawyer.