If bees could open a restaurants, I'm quite sure common knapweed - Centaurea nigra would be at the top of the menu. Why? Quite frankly, it is visited by so many species including solitary and bumble bees as well as honey bees - not to mention other pollinators.
Knapweed is rich in nectar, and flowers over a long period - it can be seen from June through to September. It's a great boon to wildlife generally, since the seeds are eaten by birds.
Although the head looks a little like a thistle, it's not a prickly plant. The leaves and stems are slender and smooth rather than covered in small, prickly spikes.
Below is a photograph of a wild verge around the corner from my house. It provides excellent sources of food (and nest sites) for bees. Included in the bounty are thistles, bramble, dandelions, clover, and hemp agrimony among others. In the summer time, I can rely on this little patch to provide some good opportunities to take photographs of bees.
Knapweed is a wildflower I would experiment with planting into my garden border if my garden was large enough. I don't include it in my garden at the moment, however, since I reckon that because there is so much knapweed in my immediate surrounding location, the bees are fairly well catered for with knapweed - a bit like dandelions.
Instead, having a small garden, I prefer to focus on including flowers to attract particular species - such as pulmonaria for hairy footed flower bees, and lamb's ear for wool carder bees. As a result, I believe I am seeing increases in numbers of these species every year, with more females emerging than in previous years.
However, if anyone is involved in a project to create wildflower verges for bees and other pollinators, then knapweed would be on the 'must have' list of flowers to include.