Perovskia (Russian sage) is the most powerful bee magnet you can find and it's both problem-free and almost maintenance free (only need to prune it once a year). Bonus point: flowering period is extremely long so you don't have to plant anything else.
But if you want Bumblebees (they do 70% of the work while bees get all the glory) nothing beats Buddleja "Flower Power".
