Do you start from seed or buy your basil as plants? I ask because some of the big meglo-marts that sell plants that have been mislabeled and sometimes seed that is not of good quality.

I would suggest first getting seeds from a reputable seed vendor and starting it yourself. It's easy to grow. I like heirloom varieties of basil best but that is a personal preference. Then look to where you are growing the basil. Perhaps your soil or conditions where you have it growing are too harsh for the basil. When I've had bitter veggies & culinary herbs it was because the soil was poor and the plants didn't get enough water.

FataMorgana

The message from Fatamorgana is really spot on. Some of the plants that may be labeled sweet basil may if fact be quite different. Basil cross-pollinates very easily and so this also creates problems with identity. Richter herbs seed catalog or Nichols nursery would be two great sources of multiple varieties of basil. Look for sweet or Italian or pesto making basil. Avoid Thai or asian forms because these are usually very strong with the anise flavor. Seeing as you are in zone 9 you could still get a few growing in pots and bring them in when the weather gets below 40-45 degree range. They really do best in the 60-90 degree weather range and like lots of sun tho your summers may call for some afternoon shade.
Happy Growing David

If you buy as a plant, crush a leaf and sample the smell.

Some basil varieties (true basils) have a strong overtone of licorice/anise.

http://en.wikipedia.org/wiki/Anethole