Which explorer is credited with reaching America in 1492?

Christopher Columbus, and every word of that sentence needs qualifying. He never set foot on the North American mainland, reaching the Caribbean and later Central and South America. He was not first - Norse settlers were at L'Anse aux Meadows in Newfoundland around AD 1000, which is archaeologically proven, and tens of millions of people were already living there.

He was also wrong about the thing that made the voyage possible. Educated Europeans had known the Earth was round since antiquity and had a decent estimate of its size; Columbus used the smallest figures he could find and concluded Asia lay about 4,000 kilometres west of Spain. It is roughly 20,000. The experts who told him he was wrong were right, and he survived because an unexpected continent was in the way.

What followed is the part that matters. His governorship of Hispaniola was brutal enough that he was arrested and returned to Spain in chains by the Crown's own investigator. The exchange of crops, animals and above all diseases that began in 1492 reshaped both hemispheres - potatoes and maize transformed European diet and population, while smallpox and measles killed a majority of the people already in the Americas, in many regions before any European arrived to see it.