Which fruit grow the fastest and easiest which produce the most grams of edible fruit per square metre?
An interesting question since you';d also have to consier what you call fruit an the length of it's cropping season and conditions of growing: toms, cucamelons, strawberries, other melons, courgettes etc. For out of greenhouse grwoing you also have to consider time to mature to cropping size and yield v predators/pests. Victoria plums crop heavy with wasps, cherries and the birds get the lot if not netted. Consider setting up pits to grow figs if your weather is half decent..but you won't compete with imported although fresh picked figs are..just superb. For outside and very hardy consider honeyberries.
For greenhouse grown 'fruit fruit' - if really talking semi tropical you can't compete but you can grow for yourself. I have citrus..more lemons than i can eat and one of the few that flowers while fruiting. Limes crop heavily but the rootstock on my grapefruit is too dwarfing for heavy crops and my best otange died a few years ago but gave me a dozen a year. I growthem in pots and take them out for the summer.
With enough headroom and winter heating then you can go madder. it's perfectly possible to grow bananas (I did once) or perhaps pomegranates. Pineapples take too long UK but easy enough (3yrs) There's lots of passionfruit varieties some will do well outdoors and c rop well. Indoor Kiwi would be more reliable than outdoors.. but probbaly best to stay simple and grow dessert grapes