I've read loads of articles about plants for pollinators. Buddleja, lavender, primroses, eryngium, ivy, honeysuckle, borage, sunflowers and single dahlias are often included, most of which I have in my garden. But in my experience these are largely ignored in favour of my hemp agrimony (Eupatorium cannabinum).
First let me quell any concerns about the name. This plant is part of the Asteraceae family and is not related in any way to the cannabis sativa cultivars which share the name "hemp".
Hemp agrimony is a tall perennial which grows wild in grasses, marshes, fens, wet woodlands and on riverbanks in the United Kingdom. We "inherited" a clump when we moved in to our current house, it probably originally self-seeded from a wild plant, but having seen how much the bees and butterflies love it we have kept it ever since. Its natural habitat suggests that it needs plenty of water, but I seldom water mine - only when it is visibly wilting - so once established it seems reasonably resilient. Every winter I cut it down to ground level and every spring it shoots back up to around 1.7 metres / 5 1/2 feet high (most sources say 1.5 metres but it always exceeds this height!), forming an attractive plant bearing slightly fragrant clusters of small pink flowers from summer right through to autumn. It is very vigorous but I've found it fairly easy to control by cutting off spreading roots with a spade. The RHS plantfinder says it prefers chalk or sand but I grow it in clay and it seems very happy there. I grow it in full sun but apparently it is also happy in part shade.
In the gallery below you can see someof the many different pollinators it attracts.