Tomatoes and chard, because they keep producing over such a long season. Swiss chard is my absolute favorite thing to grow, because unlike the tomatoes, no insects or diseases ever bother them, they are dead easy to grow and one 8' row provides all the chard two of us can eat, from April to Nov. Amazing!
Third for me would have to be basil. Another very productive, amazingly useful plant. Last year not only did I dry jars of basil, and make and freeze bunches of pesto, distill basil essential oil for use in soaps and medicinally, I also made green basil jelly and purple basil pancake syrup. Got rave reviews on all of it... what else could you do so many different things with!?
(PS herbs as a group is cheating! Can you see why they say tomatoes are the gateway drug of the gardening addiction? )