Might & Magic VI without any doubt.
The map is probably tiny by today's standards(but it's not even full of water to enlarge the map like many recent games

) but it's:
1)VARIED, there are all kinds of environments, pyramids, snow lands, dead lands and so on.
2)FULL, every step you take you find an object, a place of interest, an army of enemies, a dungeon, a city or whatever, trust me it's a joy to wander in it.
3)OPEN and "unbalanced", you can go wherever you want from the beginning, even in the most difficult dungeons if you are able to reach them, enemies and dungeons difficulty isn't proportional to the distance from the starting point, in fact one of the most difficult dungeons is really close to the initial town, you can understand how hard is a dungeon only by entering in it at your own risk.