For indoor flowers, I grow orchids, so I am obviously partial to them. My favorite orchid is a blue (Norton-line) Phalaenopsis violacea var. indigo. I bought three of them years ago from the Nortons, unbloomed, so I didn't know which color they were going to be. I have 2 blue ones, and the 3rd remains stubbornly unbloomed.
For outdoor flowers, I love roses. Can't grow them to save my life, but I like looking at other people's