Pokemon DP came out seven years ago in Japan today. Seven years of Arceus. I maintain that it has the best overworld in a Pokemon game after GS.
I think they each have their own strengths and weaknesses for the most part. BW and BW2 are probably straight up the worst; they're just overly linear and the routes never feel like they have the scale of earlier generations. DPPt has an absolutely excellent overworld design, length and variety, yeah, but is somewhat hamstringed on occasion by the overuse of HMs as obstacles. HGSS is pretty perfect. I mean, in terms of the level progression and so on, it's a bit borked, but the actual overworld itself and ignoring how the structure is placed on top of the overworld, it's juts the best for me, by a long way. RSE I find difficult to make my mind up on. I think it actually has the best early-game overworld - all of the game up until Norman is literally home run after home run in terms of pacing and overworld design. After that, it starts slipping a bit, with the routes up towards Lilycove being overly busy and a chore to navigate. Finally, the end-game just collapses entirely into Wingulls and Tentacools. Definitely goes from hero to zero. FRLG is excellent in terms of non-linearity and scale, but there's just not enough variety - everywhere is pretty same-y after a while.
I think I'd rank overworlds something like this:
HGSS > DPPt >>> RSE > FRLG >>>>> BW/BW2.