Mainly cereals - for food and spirits alike: in eastern and north-central Europe that would have meant predominantly rye. In the west and south wheat was the preferred foodgrain, though barley and oats were also consumed, especially among the poor (wheat being more expensive): barley was of course also used for beer and oats as feed for horses and oxen. Just about anything that might me made into booze probably has been at some point, though potatoes never really took off as source of spirits even when they were Ireland's staple food. Vodka's still predominantly made from grain, though now maize or rice get included, the first an introduction from Mexico and the latter little known outside southern Europe until the last few centuries.