Wow, I'm surprised at the opinions of SDN because although all the schools listed here are great, I hear people praising UCSF and UOP over UCLA and USC, respectively. My dentist graduated from UCLA and her sister (also a dentist) graduated from UCSF and both are great at what they do.
Also, I go to a pretty Pre-Dental heavy school and last year no one I knew got into UCSF while 2-3 got into UCLA. So to answer OP's question, I think UCSF is hardest to get into (though it varies per individual of course) while USC might be easiest, probably because it's just so expensive.