OS Kanto and it's not even close.
The Indigo League series was just 83 episodes long. In that time, Ash won eight badges (including three rematches), caught seven Pokemon (and released three of them), and participated in the Pokemon League (which was seven episodes long). Despite all that, they managed to build episodes around Misty, Brock and Team Rocket on top of the usual filler. You didn't have to wait too long until the next big story beat. At one point in that series there was a release, a gym battle, a capture, a rematch, another capture, and another gym battle in consecutive episodes. You never see that these days.
This isn't that OS Kanto had "good" pacing, per se. However, it benefited considerably from being a shorter season than all the rest. As Pokemon started getting longer, it found pacing itself a struggle because it didn't have enough meaningful content to fill all that new time. Total up all the so-called "meaningful" episodes in any given season and you'll find it covers just a fraction of the overall episode count. With a longer season, this would result in gym battles being months apart instead of weeks.
To get into the second part of the question: a "well paced" show depends on the type of story you're telling. Pokemon is a big, sprawling adventure, so it's pacing should actually be quite slow. The viewer should be able to take in all the sights that Ash sees on his adventure as that is the whole point of the journey. That can't be accomplished if the story is moving from beat to beat too quickly; if they're only spending half an episode somewhere before moving on.
But this only works if there are big events along the way. If there's a year of slow pacing, people will get bored. That's why there should also be several high-intensity, quicker events. These are you gym battles, your evil team plots, etc. Quick pacing doesn't give you much room to breath or reflect but it creates tension and excitement. In Pokemon's case, every gym battle should be an unbroken chain of important episodes, from the arrival to the location, to the build-up, to the actual fight itself.
What I think must also happen - and this is something Pokemon typically struggles - is that the focus should narrow the deeper into the series we get. The start of any adventure and any scene should be broad and open with possibility, but as we get to the climax there should focus on a specific thing. In this case, it's usually the Pokemon League. Once Ash has qualified, it should be full speed ahead. But Pokemon will typically go back into slow adventure mode after his eighth badge. This creates a meandering effect; a feeling that nothing's happening and the show is dragging its feet. You'll recall that Johto was infamous for this. It inevitably means we always go into a league feeling a bit cold.