I'm probably more of a Truegenner when it comes to the Pokemon spread, though I'm all over the place with regards to either the main game or region construction.
Gen 3 and 5 have some of my favorite Pokemon, but I'd still probably say Johto has the best constructed region in terms of layout and being able to traverse it, followed closely by Unova solely on the basis of having no team restrictions for main story due to HMs.
Kanto just felt very slow going because of the boring caves, and many of the routes were padded out with trainers that could not in anyway be avoided (Route 13 and 14 are awful in that respect), though I did like the somewhat roundabout design when it didn't involve backtracking.
Johto was good about not requiring too many horrible HMs (though it has one of the most obnoxiously useless HMs in Whirlpool), and the game is pretty good about giving you Fly before a point that would likely result in backtracking. Kanto's inclusion in Gen 2 though was pretty empty, mostly just walk down the non-challenging routes to beat the Gyms so you can get to the actual post-game Mt. Silver. HG/SS added some more, but still not enough for me to say it was a Gen deciding addition. The leveling curve also made the game pathetically, easy, which led to the AI usually depending on rather cheap tactics to up the difficulty (Morty having a Gengar at the 4th Gym, Claire and Lance's Dragon types at a point with virtually no viable Ice types to use) or padding the length out with the overabundance of Rocket Grunts carrying parties of 5 Rattatas.
Hoenn's big issue, though lighter on the HMs for MOST of the game, was being very roundabout and the overabundance of water routes, which turn the last part of the game essentially into a cave in terms of encounters, and make training anything Water Weak quite a bit more tedious. Not to mention needing a lot of HMs for some of the dungeons like Strength, Dive, and Rock Smash for the Seafloor Cavern, and Waterfall just feeling like an arbitrary way to stop the player from reaching Evergrande too early (there are maybe 7 Waterfalls in the game). Some of the set pieces were pretty neat though (the desert, despite my complaints the Underwater areas were cool).
Sinnoh, as much as I love some of the Pokemon, was AWFUL as a region. Mt. Coronet was oversaturated with HM needs, to the point that if you didn't have a water type to carry Surf and/or a decent Normal type for Strength/Rock Climb, you might need to bring 2 slaves (I feel like Bibarel being compatible with so many was intentional). There was also just a lot of backtracking as early as after the first gym, making a circle around the East half (skipping Fantina for some reason in DP), and of course the infamous snow routes. The Pokemon selection for main game was also horrible; besides the infamous Fire Type issue, I still can't remember more than maybe 3 fighting types you could have before the game ended.
Unova I appreciate the simplicity and the story, but the region itself is pretty simple all things considered. Very linear path with very few caves or dungeons to navigate, interesting or not. It does make the "Point A to Point B" travel much easier on replays, but I don't find myself re-exploring anything on a repeat playthrough or old file.
Kalos I just have no opinion on. Very little opposition in progression, but no particularly confusing or tedious dungeons. Only real gripe I guess I have is the oddly spacious gap between Gym 1 and 2.
That said, in terms of the gameplay, I'd say that Gen 2 felt abyssmal. The Pokemon on the whole were underpowered because of the stats and mechanics (Sneasel...), so the game felt like a drag because despite being 10 levels above the trainers at times, I found very few battles that actually ended smoothly because so many Pokemon took 3-4 moves to kill. Gen 4 was when I found the story going a bit faster because if I used a Gengar, it could actually play on what makes it good to help me finish the game faster. Being able to use everything the way its stats dictated without the limitations of a poorly thought out movepool for the Physical/Special split made it possible for me to experiment instead of always inevitably having to go back and use Mareep or Magmar for the 12th time in Johto.