Anethole and estragole are the compounds behind the liquorice note in fennel, tarragon and dill, and each plant carries a different balance of them. Fennel is the sweetest, tarragon the sharpest and most peppery, dill the greenest.
French tarragon is the interesting one here: it cannot be grown from seed, is rarely sold fresh, and makes an infusion almost nobody has tried. It also numbs the tongue slightly, which is estragole doing exactly what it does.
