My reason: the combination of great clinical care and research opportinities coupled to the amazing city of SF. You want to be surrounded with amazing people as well as amazing things to do when you're not in school (theaters, shopping, pro sports, golden gate park, Sony Metreon, you name it). If you want rankings, UCSF has the best combined rankings for clinical and primary care in the country (only Baylor is even close ~10 ~10 vs ~5, ~5). Plus, having world class pharmacy, nursing, dental, physical therapy, etc schools nearby means there are lots of intelligent and driven people to meet and hang out with.
However, Stanford represents the 'ivy' league of the west coast with renown undergrad departments and a research intensive med school. Reputation alone (if you don't count their football team) accounts for Stanford even being comparable to UCSF. Plus, it has the palm tree lined boulevards of Palo Alto and the care free northern CA attitude (that appeals to some). However, the clinical sites as a med student do not compare to the UCSF med center, SF General, VA, etc. Although I'll admit that Lucille Packard Children's hospital has a very good reputation.