Dark purple is not my favourite colour of petunias, but I really enjoy their spicy fragrance so I usually have some in a pot on the back deck so I smell them on the way by.
Heliotrope has a lovely sweet fragrance, like vanilla to my nose but I have read that some people find it smells like cherry pie. Full sun, moist rich soil will keep it growing optimally, and giving you wonderful wafts of perfume. Warm late afternoon sun is especially good for releasing the fragrance. It's not always available at the garden centres but it is fairly easy to start from seed in early spring.