Probably already named, but Pirates of the Caribbean: Dead Man's Chest and At World's End. Also, Settlers 6 > Settlers 5, 4 and 3 (though not 2 or 1). And perhaps you can also say that Warcraft 3 > Warcraft 2 > Warcraft: Orcs & Humans, though all three were masterpieces for their time and can't really be compared properly.