For outside of UK, I'd have to say my flight down over the myriad of islands dotted along the Croatian coast was pretty spectacular, enhanced by the views of the Old City of Dubrovnik which contrasted sharply with the lush greenery of Montenegro.
In UK, I enjoyed flying down the south west coast over Plymouth (note to self: do not land there, not even in a Jodel ;-)) past the Eden Project, looking at the emerald green water near to old mines, then flying past Perranporth to Newquay, seeing the deserted beaches in mid summer......all the time wondering why people fly to Spain, to be crammed in like Sardines on the beach at Benidorm.....