I'm going to go with Unova here. Unova has a reasonable level curve, unlike the Kanto, Johto, Hoenn, and Sinnoh E4, and it has good rematch teams in BW1 and BW2. Its type selections are also an interesting combination, even if the initial teams are lackluster in BW1 and BW2 non-Challenge mode prior to rematches. However, Marshal's team, and to a lesser extent Caitlin's team, are kind of lackluster compared to the other two.
In theory, I like USUM Alola's E4 more, but I haven't actually played Alola yet so I can't rate them, since I'm not sure how they actually function as characters rather than just abstract teams.