Even picking a handful of "best" overworld themes is hard for me.
Including games where you don't actually manually walk on the overworld, but it is a distinct overworld with its own theme (you just select locations instead of walking across them manually)

- Chrono Cross' four overworld themes are all top notch, though my favorite of them is Shore of Dreams (aka Dreams of the Shore Near Another World)
- All the Wild Arms games' overworld themes, hard to say which one among these I like most, probably either 1 or 3.
- Golden Sun 1 and 2, my preference being GS2's second overworld theme (Marching Forward with Determination)
- Ni No Kuni 1 and 2
- All the Ar Tonelico games and Ar nosurge (preference here probably goes to Ar nosurge's "Green Lands for Ar nosurge")
- Legend of Dragoon (second world map theme in particular)
- Lost Oyssey as well, as Fox mentioned.

As for worst..I agree that it's a tough one because overworld themes tend to be good even among otherwise mediocre/bad soundtracks. Like even Dragon Quest 11's world map theme isn't something I'd actually call bad, though it's probably the worst one I can think of right now.