Dahlias. I think this is one that I dug up and replanted this year — as they get older, the corms get bigger, and in my experience, the plants get bigger, putting out more blooms. One more reason to go to the trouble of digging them up and storing them, rather than just buying new corms every year.
Also, as they get bigger, you can often divide them (in which case, you go back to smaller plants with fewer flowers the next year) to share with friends or spread around your garden. Budget-friendly!