The correct answer is "an unknown Paleoindian more than 16,000 years ago." If our choice is limited to Columbus versus the Norse, though, the Norse win. They reached Greenland (which is part North America) in the late 10th Century. In 1000 CE, Leif Erikson led an expedition to mainland North America after hearing about land being sighted to the west by sailors who had been blown off course (the earlier sailors, led by Bjarni Herjólfsson, didn't land there though). The Norse had some off and on again contact with mainland North America and more temperate islands to the south (notably Newfoundland, where a Norse archaeological site - L'Anse aux Meadows - has been discovered dating to this period. Ultimately, they didn't put down any permanent roots and headed back to Greenland.

The Norse themselves may have been beaten to the Americas by the Polynesians. Throughout a large portion of Polynesia, sweet potatoes have been a common crop for hundreds of years. But sweet potatoes are native to South America, and the words people in Polynesia use for the plant are essentially the same as those used in the Andes (allowing for some linguistic shift as the word passes from one community to another) - it's kumara in Rapanui, the language of Easter Island, and kumar in Quechua. There's also evidence for pre-Columbian genetic admixture between people of Easter Island and people in South America. Unlike the situation with the Norse, we don't know exactly when (though based on the spread of sweet potatoes, it couldn't be after Norse contact with North America, but may be contemporaneous) or where Polynesian contact would have occurred, or really which direction it went in (maybe it's actually people from South America heading into the Pacific, rather than the other way around).