I wonder which of the campaigns is the original first one you can choose in the menu

The Banewars is the first campaign from Reign of Heroes; the other four came in Darklords Rising. Bane's Revenge is the sequel campaign, The Horn of Kor is set in historical times, Drak-Dum is its own mini-campaign and Thalassia isn't set in the same world as the other campaigns from a lore perspective.