Most things that grow underground are safe from theft. Such as potatoes (which many people don't recognize by the plant, anyway), sweet potatoes, jerusalem artichokes, carrots, garlic, turnips, rutabaga, and parsnips. Herbs are also left completely alone in my experience. Most people don't know what they are or what to do with them.

In fact, in our garden which is completely unfenced, we're having little or no problem with thieves.

Traditionally, tomatoes are attractive to thieves of all kinds, human and animal alike. I think it's because they are so noticable even from afar. You can get around that by planting unusually colored tomatoes, though. Yellow ones, for instance, that you pick before others are aware they are ripe.

Cabbages draw a lot of attention but because they are large I haven't had any thefts of them so far. Last year beans and peppers were common items to be stolen from the garden I belonged to along with tomatoes and fruit, and again, I think it's because they are commonly recognized and easy to see from afar.

I remember wishing, last year, that more people would steal zucchini. Late in the summer I even began draping the vines over the fence in hopes that they would do so.

Our garden is locked and only 5 gardeners right now. And most of those are too lazy to even pick their own ripe produce!
Tally Ho!