What are you looking for? Pretty much anywhere between the presidio neighborhood in sf all the way down the peninsula to say Cupertino/Sunnyvale will be good, depending on what you want.<p>BART stops running about 12:30 AM and caltrain stops around 9, so unless you plan to work normal hours, plan on needing a car or a bike to get to and from the office. Commuting in a car is not fun, because traffic can be pretty bad.
Recommend trying to find something short term in SF. Apart from the fact that all the major startups (Twitter, Obvious Corp, Pinterest, Airbnb, Square, etc) are based in SF, it's only a 35-40 min drive to Mountain View and most importantly, you'll be able to connect more easily with YC alum mostly living in SF. Check Craigslist SF Bay for short term, furnished housing.
As bad as mountain view is, you'll want to be there for at least the time during YC.<p>IIRC, YC will send you leads on places to live shortly after interviews.
South Bay for sure, during YC. MV, Palo Alto, Los Altos, Menlo Park etc. You can move to SF after YC.<p>Anywhere close to YC with easy access to the 280 is best.