That is a real difficult question as I love most summer and stone fruits.
Once upon a time I would have said strawberries however the last few years the strawberries taste of little, and I have virtually stopped buying them, as they mostly do not last long either.
Perhaps I would say water melon - it is crispy, sweet and doubles for me instead of a salad with some meals. It is low calorie and refreshing and is therefore one of the more versatile fruits.
Lychees!!!! Definitely lychees!!!! I also love cherries. Both of these have very short seasons, so I would have to say the I really love white nectarines as well!!! I eat such a lot of fruit al year through, but the sumer fruits are my favourites I think due to the varuety! In winter here in Nth Qld, I adore the really fabulously sweet strawberries which whe can always find so plentiful and cheap!! I grow huge passionfruit all year round, so I cannot really call these a Summer Fruit though they definitely are a FAVOURITE!!
A tasty ripe but not over ripe apricot just plucked from the tree in someone's backyard. However, I rarely get one of these so I am going to plant a dwarf apricot tree in my backyard and wait patiently. The fruit from the shops isn't tasty so I rarely buy 'fresh' apricots and have to make do with tinned or dried fruit.
I love fruit and with global cultivation a reality, it means I can experience my favourite fruits at any time of the year (albeit more expensively). However, I can't go past watermelon, pineapple, peaches, plums, apricots, blueberries, cherries. These fruits are always in our fridge during summer.