My favourite flowers are those of hoya carnosa compacta. They aren't always grown for their flowers as they can be difficult to bloom. They're very pretty and I'd like to try to get one to flower for me one day
This may not count but I really like begonia varieties such as escargot. These cultivars are grown for their gorgeous leaves but I think that the flowers are nice as well :)
I also like periwinkles, violets and orchids to name a few. And petunias, they've contributed a lot to science and I think they're pretty. I like all plants so I feel any answer to this question is a good one