If the answer is to be close to the beach and the downtown, then Montgomery is one possibility. You will get a range of ocean views, including whitewater, from different streets, so depending on what is important in terms of view, you can select from a variety of streets offering different views. It can vary from the east side of a street to the west as well as from house to house. If the "best street" in your mind is the most expensive, that will be another street or two. Ultimately the best street is the one that meets your needs at the price you are willing to pay.
To answer that question the right way, it all depends on if you want privacy or ocean views and close to the ocean.
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I am not sure you could rank a single best street but I would avoid "San Elijo" because of the excessive traffic noise. Montgomery, Oxford and Cambridge St certainly ranks up there with beautiful homes and views. Traffic is far less. Cardiff is a beautiful area and I am sure you really can't go wrong. Seaside Market and VG's bakery are great places to go. Hope this helps.
To be serious though...surfer=san elijo (check cardiff reef from your bed), gentleman farmer=crest drive (some great larger parcels AND the view, international man of mystery (like Dusty Brazil) =composer district.