Roses and pansies are my favorites! It's so hard to choose since cosmos are sweet too, especially for wearing. I really like planting roses around town though so they're my absolute favorites - I'm just hoping to successfully breed blue ones soon.
I'm planning on filling my town with only roses and lillies. I think the plain white lillies look best, but I love all the hybrid rose colors, especially the black ones(having trouble getting any to grow though, but I'm a lazy gardener).
I adore Pansies and Azaleas in real life, and both happen to be in Animal Crossing New Leaf. So I consider myself pretty lucky! Granted one is a bush, lol.