Almost all the responses so far circle about the eating qualities as an unprepared fruit. There are some dips into nutrition and playing with the definition of ‘fruit’. Rhubarb is great but a stem eaten as a dessert and tomatoes are botanically fruit (technically berries) that are treated like a vegetable in food preparation and (in some places) legally also.
How about a fruit that is used in many cuisines, that is essential to some classic dishes, where every part but the seeds can be eaten and whose aroma evokes cleanliness in many cultures but is almost never eaten unprepared?
For total flexibility, all-round pleasure and usefulness you cannot go past the lemon. Google “recipe lemon” for half a billion hits.
Lemon juice is a key sour flavour that adds depth and complexity to dishes both savoury and sweet, it is more flexible than vinegar and more common than other fruit-based sour ingredients like tamarind. Sour is very important to taste being one of the big five flavours that have been shown to be detected by our taste buds: sweet, sour, bitter, salt and umami.
The zest (just the yellow layer of the peel) contains a mix of natural essential oils that most of us find very appealing. These oils are essential in the sense of being an essence not of being obligatory. Desserts and patisserie would be much less interesting without lemon zest (and juice) and who could forget the cleaning products that tell you that you are a good homemaker using them in your house because they smell of lemon. Advertisers found that odours pull on emotions long ago.
What about the white part of the skin, you can’t eat that. Yes you can, here is how.
This condiment is most often encountered in north African cooking as an accompaniment to dishes like targine. You can buy it in speciality stores for silly prices like $10 to $15 for a small jar or make it yourself for much less.
You need a large wide-mouth container with a lid, preferably glass, enough lemons to fill it plus a few more and salt. If you cannot grow your own do your preserving when lemons are cheap and plentiful. I grow and prefer Meyer lemons but you cannot always get them commercially. I only do this every few years when there is a bumper crop.
Wash and dry your lemons, keep aside some for juicing. Slice longways into quarters discarding any damaged or blemished segments. Pack them tightly into the jar in layers and sprinkle each layer generously with salt. Add juice as you go shaking to remove air bubbles. Continue filling until the jar is nearly full and cover the fruit with juice. The fruit will tend to float so you may want to put some inert object like a small plate in the top to hold them down. Any that are not submerged will not preserve properly. Put the lid on not quite tight and put the jar away in a dark place where it will not be disturbed.
Depending on the temperature and the amount of salt you use the fruit may ferment somewhat and produce gas. During the process you may get a white bloom on top. It is a harmless fungus just scoop it off. After three weeks tighten the lid. Keep for at least 6 weeks before using.
To use, wash the quarters under running water and peel the pulp off the skin with your thumbnail, it will come away easily, discard the pulp. Wash and dry the skin and slice thinly. Use to garnish any dish where lemon will work. Usually used in savoury dishes but there is no reason it won’t go with sweet dishes given some thought. The skin ends up slightly salty, slightly tart but bursting with lemon flavour.
For the first few months the preserves will remain white-yellow but with time they will brown. The liquid will also darken and eventually turn to a jelly. Sadly it is on no use as it is too salty. I have kept such preserves for many years and they remain good to use. No harmful microbes can grow in the medium which is very salty and very acid. You may get the tops poke out of the liquid and turn nasty looking, if so just discard them the rest will be fine.
So there you are, the lemon, the one fruit that cooks cannot do without.