What's the best place in San Francisco to fly kites?

Ocean Beach, fo sho’.

Crissy Field.

+1 Crissy Field

Ocean beach is OK, but a lot colder in general, and the waves can be a bit iffy in the winter.

Another good place is Marina Green – http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=Marina+Green,+San+Francisco,+CA&sspn=0.009015,0.01929&ie=UTF8&hq=Marina+Green&hnear=Marina+Green,+San+Francisco,+CA+94123&z=14

In east bay, Cesar Chavez park in Berkeley is an excellent place to fly, and there’s frequently lots of other people flying kites.

April or May through August or September, the winds pick up around 1–2pm, building through the evening until about sunset. The winds are typically dead in the mornings, so you’ll have better luck if you go out mid-afternoon.

The winds are iffy in the winter, but if you go out the couple of days after the rain moved through, you should have good (if a little strong) winds.

And lest I forget, there’s this map: http://sfports.wr.usgs.gov/cgi-bin/wind/windbin.cgi which can help you figure out where is good to fly. In general, for kites, green arrows mean good, yellow or red arrows mean too windy.