I wouldn't vote for Ibn Battuta because while being very well travelled, he really did not discover anything. He basically visited all the places that Muslims already knew about and already we're living in. An impressive personal achievement and it makes for an interesting life story, but it didn't really change anything.
Everyone else seems to have actually either discovered something brand new for civilization at the time (the Carthaginian voyages, Leif Erikson), or made the first real contact for their civilization to another one they may have known about it in very theoretical way, but was for all real purposes the first contact of their civilization (Marco Polo, Ibn Fadlan) and expanded their civilization's knowledge base immensely.
Everyone else seems to have actually either discovered something brand new for civilization at the time (the Carthaginian voyages, Leif Erikson), or made the first real contact for their civilization to another one they may have known about it in very theoretical way, but was for all real purposes the first contact of their civilization (Marco Polo, Ibn Fadlan) and expanded their civilization's knowledge base immensely.