There's lots of lists of flowers to plant to help the bees out there, but this is the first time I've seen a scientific comparison of the attractiveness of different plants.
Figure 3 from the paper seems to offer a quick glimpse at which types of flowers were the favourites.
I've been interested in agastache (anise hyssop), since reading praise for it in beekeeping literature a few years ago and because it's native to North America. This gives me a little extra incentive to try planting a little more of it.
I sent Mihail a question and he was quick to remind me that this study was only conducted in one area and results may vary in different regions. So you may want to take this research as inspiration to try setting up a test garden near your own bees and see how your results compare.