I should just point out that Happyentropy's rankings are specific to Algebra/Number Theory/Algebraic Geometry, as he stated. It is really important to talk to faculty to find out who is good for what. You'll notice that Chicago, MIT, Columbia are all below UMich, but in terms of general respect/esteem this is not the case.
It is generally believed that the top 6 programs are Princeton, Harvard, MIT, Stanford, Chicago, and Berkeley. However, if you want to work with John Baez, and he is at UCR, then it is something to consider, but if your interests shift you could be screwed.
I thought the following blog post was very informative:
Congrats, you’ve got PhD offers… now what? « An American Physics Student in England
It is written by a physicist, but the lessons are generally applicable.