Florian, I bet you can grow and fruit different varieties of rowan ( European mountain ash), or sorbus aucuparia. I make jelly out of mine. They may even grow wild where you live. I'm growing the Native American rowan and a Chinese pink-fruited variety, and a white fruit variety. I'm sure you already know rowan is the ultimate fruit tree for cold long winters and short cool summers.
Lol, rowan (known here as reyniber) is an Icelandic native; for example, one of our more popular southern beaches is Reynisfjara, aka Rowan Beach. They can definitely handle continental Europe!
I made a jelly from it once (freezing and a long cooking as I found instructions for online), and found it... a bit of odd. It tasted like something you might serve with meat, more than something you'd spread on bread. Yes, sweet and with fruit tastes, but also something else that I couldn't put my finger on.
Then again, I'm not really a jelly person to begin with, and the need to process the berries to remove toxic parasorbic acid was sort of a turn-off to me. I understand that some varieties have actually been bred for consumption, so maybe they're better. On the upside, rowans sure are pretty trees, esp. in the winter.
When one asks about the most exotic fruit, are we talking taste or appearance, and by "exotic", do we mean "tropical" or "unusual"? Lardizabalaceae has some weird looking temperate edible species, for example - often strongly lilac-coloured fruits that open themselves up when ripe, revealing their mucilaginous white interiors. Sweetness is often quite high, but acidity is usually low. A really remarkable Lardizabalaceae species is the monotypic Boquila trifoliata ("Chameleon Vine") which has the so-far scientifically unexplained ability to mimic the leaves of whatever plant it grows on (even fake plants), in colour, size and shape. It's so good at what it does that its ability went undiscovered until relatively recently (scientists had just assumed that the species was incredibly variable, not noticing that it always matched to its host and would change leaf styles as it grew between plants). The berries are reportedly edible but I haven't found anyone who's actually eaten them. Supposed to be tolerant to 7b.