Home Owner, Anza Vista, San Francisco, CA
It's been a long time since I've used Greyhound, but when I did I boarded at the Embarcadero. If that's where you mean, Embarcadero station is closest. Looks like they are now at 200 Folsom, in which case Embarcadero is still closest.
If they have a 7th Street stop, Civic Center is probably closest.