What was the most important food staple and field crop in Europe before the introduction of potatoes?

Various grains - wheat, barley, rye and oats leading the field. Local growing conditions tended to dictate which was the preferred grain, which would generally be made into porridge or bread. Or beer. Wheat was considered the finest grain, but was also the hardest to grow, hence the most expensive, and not widely eaten by commoners.

https://en.wikipedia.org/wiki/Medieval_cuisine#Cereals

I'd actually say that barley alone was the most important food (and drink) crop up to the early modern era (which began with the 16th century) and for millennia. Among the Greeks and Romans and other, far earlier, ancient peoples it was an extremely important crop and staple.

In the 16th century, barley's importance was supplanted (pun intended) by that of wheat, but for most of agricultural history, in Europe and elsewhere, barley was king.

https://en.wikipedia.org/wiki/Barley