Despite Felipe Fernandez-Armesto's belief (expressed in the excellent "Near a Thousand Tables : A History of Food") that salad bars are "repulsive", I'm pleased to report two winners in my book.
Fountain Cafe @ Crocker Galleria
Quite fresh, never too crowded, and reasonably good selection. Lots of unadorned vegetables of pretty high quality (though once the broccoli tasted like cigarettes, which I still can't figure out), refilled regularly. The prepared salad offerings can be soggy and seem mass-produced, so I stay away from them. But it's my favorite way to assemble a good spinach salad midday.
Napa Ranch on California btn Montgomery and Sansome.
The raw veggies aren't quite as fresh, but the selection of prepared foods is much better. Very good grain/legume salads, including a cracked wheat salad and a vinegary/oniony black bean salad. Nice cold noodles, and good firm tofu as well. More expensive, but maybe just because I tend to buy heavier items there.
Other favorite salad bars in Financial District / Union Square / SOMA?