I was at the Cincinnati Zoo last weekend and noticed one of their flowers attracting lots of attention from both honeybees and mason bees. The zoo has mason bee houses and honeybee hives.
The flower was Pentas and the variety was "graffiti lipstick." It was a red flower. The bees were all over it. After doing some online reading, it appears that there are several available varieties of the flower in several colors. From what I can tell all are abundant nectar producers and attract birds, bees, and butterflies.
It appears that it can be grown as an annual in places that have frost (as we do). I am sure they keep them watered and cared for well, but even in an unusually hot September the flowers looked great!
In Florida Pentas are a very popular flowering shrubs, mostly because it can survive our crazy temps. I can attest that bees and birds love them but it does seem to vary by color. I have had the best success with the red variants followed by the whites. The pink and purple variants seem to have less of an attraction. They are perennials but get very leggy and we usually replace them every year for aesthetic reasons. Mine are usually covered at any time by a combination of my honeybees and local pollinators.