Choose only unsprayed organic flowers, whether they're from your garden or a farmers' market. Avoid commercial flowers from florist shops or supermarkets; they've been sprayed with pesticides. If picking flowers in the wild, avoid those by the roadside, as they may have been sprayed with herbicides or have absorbed car exhaust.
Common Edible Flowers
culinary herb flowers, such as basil, borage, chamomile, and chive
honeysuckle (caveat: its berries are highly poisonous)
pansy and viola