Canna lilies. They have the highest percentage of starch of any known tuber, they grow and spread rapidly, they will grow in wet/dry/sunny/shady areas, they are not bothered by pests, they are pretty and most people won't recognize them as food sources. They don't need to be fertilized, weeded, or sprayed with pesticide.

You cook them the same way you would cook potatoes (boiled, roasted, baked, french fries...but for some reason they don't make crispy hashbrowns). In So. California they grow all year long. You can leave the tubers in the ground until you want them or keep them in a root cellar, you can also dry them or turn them into flour.

I believe 90% of one's survival garden should be devoted to calories. It easy to get the vitamins and minerals one needs, either through stored vitamin tablets or any number of wild edibles, but calories are very hard to come by. They are also the most critical thing you'll need in a long-term survival situation. Pretty much any other type of plant is just flavoring in my opinion.

Also get a copy of either Gaia's Garden 2nd Ed. (my favorite gardening book EVER) or Square Foot Gardening. Both books are really good but have very different takes on how to grow a lot of food in a little space.

I recommend you also look into edible landscaping. There are many trees, shrubs, bushes, and flowers that are wonderful and flavorful sources of food. EdibleLandscaping.com is a great place to start learning how to replace your current landscaping plants with edibles.

-Blast