This question gets asked a lot and I wish there was a simple answer. The "short" answer is: Use the one you can get easily. For most people, this means animal fat.
Then there is the long answer.
My favorite sealer is pure tung oil. Is it the best? No. Shellac is probably the best "natural" sealer.
How about beeswax or pine pitch, or some kind of drying nut/seed oil? Look up the properties and compare. You will find that the more layers you apply, the better. You may also find that mixing these will give good results.
How about blood, tree sap, plant juice, or just plain burnishing? Nope. They are period but they are not very effective.