Well, since spudster opened the door for plugs, I had better put one in for that school across the bay. 😀 The UCSF Cancer Center has been a world leader in groundbreaking research on cancer biology, diagnosis, and treatment. There is a great group of basic and clinical researchers that are people doing amazing work using a wide range of approaches. We have people like Liz Blackburn, Zena Werb, Joe Gray, Lisa Coussens, Doug Hanahan, Mark Israel, Thea Tlsty, Gerard Evan, and of course Nobel laureate Mike Bishop.
Of course I should be fair and mention some other good programs... like Dana Farber (Harvard-affilitate), Memorial-Sloan-Kettering (Cornell-affiliate), etc. Cancer is a big topic with many sub-areas of investigation. As others above mentioned, you might want to narrow it down a bit by thinking about it in terms of specific cancer types or by approaching cancer biology from a specific angle. In addition, as none mentioned, there are many other factors to take into consideration in choosing where to go.
Good luck. 😀