I'm only about to start university, so from the perspective of what's best prep for degree obviously my experiences are fairly irrelevant.
Regardless, I second Big Bang by Simon Singh, as it was great to have a more detailed look at the development of the theory, rather than just the finished product preceded by a list of names.
And the Feynman recommendations, as he's so charismatic, and he makes the normal stuff, the mechanics and so forth, seem interesting as well as the more exotic and bizarre. Plus, if you hunt around a bit, some of the recordings of him delivering his famous lectures are available online as free audio files for download.
ag