Minecraft wiki entry for Farming, associated text: Cactus provides the most experience (1/item), but a regular furnace must be used.
Each furnace accumulates 6 experience per minute. A chestful provides enough experience to go from none to level 32.
Potatoes (or chicken) provide about a third of that (0.35/item), but a fully-automated farm may be difficult.
A smoker accumulates 4.2 experience per minute. A chestful provides enough experience to go from none to level 20.
Kelp provides the least experience (0.1/item), but can be cooked in a smoker for 1.2 experience/minute. A chestful provides enough experience to go from nothing to level 10.