Kegs: Hops, wheat, the most expensive fruits/vegetables (base value, products not affected by quality) Wine takes a week to make, shorter time for other stuff.
Preserve jar: Fish roe, other fruits/vegetables, iridium quality mushrooms, for all but sturgeon roe it produces results in 2-3 days.
Smoker: The most expensive fishes you have, as it keeps the quality of the fish (the legendary fishes and those caught in the mines are better put in ponds to produce roe)
Dehydrator: Overnight results, best used on base and silver quality fruits/mushrooms.
Casks (once you have the 3rd house upgrade): Cheese and the highest value wines (starfruit, ancient fruit preferably). Gold quality cheese take a week to turn into iridium, while base quality cheese takes two weeks. Wine takes 2 seasons to go from base to iridium quality. Beer and pale ale aren't worth aging in my opinion.
Eggs and milk is best processed into mayonnaise and cheese. Wool, if you have rabbits or sheep, is best turned into cloth (iridium quality wool always produce 2 pieces of cloth, while gold and silver only do it sometimes)