Which cultivated plants produce most nectar for pollinators?
Lacy phacelia or purple tansy (Phacelia tanacetifolia), which can be
planted any time of year due to its short growing season. After
flowering, it is plowed into the ground to enrich the soil with
humus
All types of clovers: excellent nectar plants that plowed after
flowering and enrich the soil with nitrogen, or scythe them and use
as feed for animals
Sunflowers: after flowering, they greatly enhance the structure of
soil and enrich it with hummus
Rapeseed and poppy
Buckwheat: planted it in summer and when it fades, the seeds and
ground into flour


Scroll to page 13 (Appendix II). Here you will find a list of all significant nectar-producing plants in Europe

Scroll to page 21 for average nectar production (mg sugar/day/flower) of important agricultural crop flowers

Melliferous potential of twenty-seven plant families determined from the N data sets recorded in thirty-eight papers ranked by decreasing median values - scroll to page 5 under Table 1) 

The google keywords you are looking for are along the lines of: "melliferous potential of agricultural crops in europe" and variants of this