garlic isn't beautiful in flower, but the "Spanish Giant Garlic", a type of Allium ampeloprasum has beautyful purple flowers.
Onions have nice white (sometimes rosy or bluish) flowers, but you can't use them once they are in flower.
Winter Onion (Allium fistulosum) can be used as flowering plant, but the yellowish white flower heads are not as ornamental as other onions by far.

Actually, Elephant Garlic has a nice large bloom on a 3 to 5' stalk. (you can buy a bulb at most grocery stores) And the plus is that you can eat the bulb or replant the cloves for more blooms. Also many of the larger leaks have an attractive bloom. Some I have are blueish and some are white. They will produce seed so cut them off before they drop the seeds.