>>56134717Gens 1-3 all do a good job of it. Gen 4 qualifies, too.
I think it comes down to the fact that those games were willing to just shut the fuck up for a minute and let the player experience the world a little. They were a lot less handhold-y, and they didn't feel the need to keep the player amused with new colorful story events every five minutes. Also, they weren't afraid to be a bit more subdued when the situation called for it.
To me, the idea of "adventure" in a Pokemon game is exemplified by stuff like exploring the depths of the Whirl Islands to find Lugia or uncovering the mystery of the Regis. I stopped paying much attention to the vanilla games around Gen 7 so I could be wrong, but I doubt the series in its current form would be content to have a legendary encounter play out like that. It's all very in-your-face like a theme park, which I think is why the newer stuff fails to come across as a true adventure.