Ok this feels like a loaded question.. how can you choose just one?
Also, for me, each season brings a favorite, there are so many great ones, too many really. So for flowers with great scent I would have to say Freesia, Hyacinth and roses, then
flowers for long lasting on the stem would have to be Dahlia,
and the beauty of the Iris family for the variety,
Eatable plants would have to be ANY fruit, the mints (love chocolate the most) and Nasturshiums.