Which Europeans discovered America before Christopher Columbus?
Without a specific map or route which specifically documents proof that "A" long trip was made coming our way, it would be hard to determine exactly who visited us first.
Also, be careful with the word "Europeans". References have been made about Norsemen and Celts, Vikings, and other groups, even Egyptians! (....and, of course, aliens)
Now, if you want to go on the safe side, it is accepted that people from Siberia travelled through the Bering Strait (when it was frozen and people could walk over it) over 20,000 years ago all the way towards what we now call Central America and, generations later, developed massive tribes called the Olmecs, Mayans, Aztecs, and the Incas.
If you reduce your argument to Europeans, you would be talking about continental Europe, and that is made of Eastern and Western Europe. No specific information (except for the one I wrote before) is historically accepted about actual continental Europe voyages to this side of the ocean.
But that is the beauty of history: There is NO "final answer."
According to the Greenland Saga, the legendary stories of how Vikings Thorvald Arvaldson, Erik the Red and Leif Erikson (grandfather, father, and son) voyaged to Iceland, Greenland and North America. Apparently father Erik had some legal problems in his homeland of Norway and thought it best to take the advice given to him...leave or suffer grave consequences. As a result, Erik traveled to his father's Greenland, and settled a colony in Iceland. Several years later his son Leif voyaged to present day Newfoundland and Labrador in North America. In 1961 the archaeological excavation of Helge and Anne Ingstad provided evidence for the Greenland Saga. Their excavation at Epave Bay which they named L'Anse aux Meadows brought the (temporary) Viking colony to life. The remnants of the long houses and iron supplies radiocarbon dated between 990-1035-40 A.D. provide evidence that the Vikings were in America long before the Age of Exploration (Spain, Portugal, France, and England)