Old World Blues. It's a big map with a bunch of areas to explore that are not required in the main quest. Hell, you even get different endings depending on what you find.
To a smaller degree, Honest Hearts. The map is relatively small, but the terrain is interesting and more "vertical" than usual. There are also a number of places to explore, that often piece together the backstory of Zion, which is actually more compelling than that DLC's main plot.