While citrus juice, especially lemon juice, has an important place in cooking and baking, it's a bit of a one-note wonder. I get more excited about the volatile and aromatic oils found in the skin of the fruit, which contain floral and tangy tones as well as a slight, sophisticated bitterness. In sweets, zest adds a colorful counterpoint to fresh berry fillings, dried fruit compotes, suave custards, and creamy frostings. Mellowed when baked, it insinuates its sunny personality into cakes and cookies. All citrus fruits are candidates for zesting.
Lemons are the most popular.
The zest from tangerines or blood oranges offers exquisitely flowery aromas. Grapefruits yield a wonderfully complex zest. Lime zest loses some of its kick when cooked, but added just before freezing to a sorbet or granita, it can't be beat. Look for firm fruit whose skin is clear of soft spots. I also try to buy organic produce when possible, especially citrus, since I'm using the outside of the fruit.