Hoenn is by far the best Pokemon region, and it's because of all the water and the vastly varied landscapes. I don't necessarily mean it has the best Pokemon, or that Generation 3 games are the best, I just mean the physical region of Hoenn is my favourite region to be in and explore.
We all know about the too much water meme, but the water in the game is actually great. It allows for interesting cities and towns, underwater exploration, shipwrecks, and interconnects the whole region in great ways. It definitely doesn't need more water, but the amount it has is spot on.
A lot of the latter part of any Hoenn game takes place on water or underwater. Narratively, this makes a lot of sense. The Hoenn games are about the environment and why balance is important. We spend the first five badges worth of gameplay on land so it's a smart move to then put us on the water. Cycling Road is a great example of this - two twisting land paths all surrounded by water that you can explore later. The water can be annoying because water tiles are like cave tiles and wild Pokemon can jump out at you at any time. However, if you lead with a Pokemon with Intimidate you'll get far fewer encounters. Also, if you're playing the remakes then you can just spam Repels and ignore all the watery critters altogether.
Aside from the narrative reasons the water makes sense, there are also the gameplay and pacing elements that are helped by the water. Just after getting your first badge, you're blocked by a rocky cave to the east and water to the southwest. You lack either the Rock Smash or Surf HMs so are unable to progress. Instead, you have to help an old sailor who will then offer to ferry you around the region. In the original Gen 3 games, these trips speed you along routes littered with trainers, islands, and shipwrecks. They make you curious about what's around you and you'll likely make a mental note to return there when you get Surf. The remakes swap these trips for cutscenes though, which takes away some of the intrigue of the routes around Dewford Town for newer players.
Dewford is a beautiful town, excellently realised. It's got a charm that as a British person who spent a lot of his childhood in seaside towns I can really appreciate. All the houses are along the shore, but venture deeper into the island and you find a deep and sprawling cave - the game encourages exploration at every single turn. Even the obsession with trends and fads in Dewford makes sense. As the town between two shipping and swimming routes, it makes sense that people would regularly be stopping there for a short time and imparting small tidbits of something from the mainland that the townsfolk can then obsess over.
The original Hoenn games made sure we all knew that water was important by adding reflections to it, a graphical marvel for the Pokemon series at the time. Seeing the sprites reflected back in the water in Petalburg or even as I walked past a puddle soothed me immensely - it was a great way to make the water a calming presence in the environment. When walking across the wooden bridge between Rustboro and Petalburg Woods the reflections would peek through the slats in the wood. Even standing in the surf along the shore is great, as the water licks up at your feet as your footsteps fade in the sand behind you. This evokes the sensory reaction from the memory of being barefoot in the sea or feeling the sand between your fingers and toes at the beach.
There are so many treasures hidden in Hoenn's seas and rivers. You can catch Feebas - a Pokemon that has no business being as rare as it is - only in a handful of random patches of water in one specific river by the Weather Institute. There are waterfalls to climb in caves to get to hidden areas with more rare Pokemon like Bagon. There are also entirely optional areas like Pacifidlog Town or Sky Pillar that are hidden amongst raging currents. Mossdeep and Sootopolis are incredible locations - seriously, a city in a volcanic crater is an awesome idea and fits the epic scale of the game's legendaries perfectly.
The abundance of water even helps in locations where it doesn't pool. Fortree City, a city of treehouses in a lush, dense forest is probably only there due to the constant rainfall around it. And, that abundance of water causes the lack of it to contrast heavily and have much more impact. Being able to experience heavy rain, dazzling sunshine, sandstorms, and ash piles in the same region made me feel like a real adventurer exploring the world, not just one small portion of it.
I haven't even gotten into the adventure puzzle of unlocking the three Regis, or the roaming legendaries that force you to traipse all over the land in search of them, or the Weather Institute alerting you to Kyogre and Groudon in Emerald, or the different secret bases you can get. The very environment is yours to play with in ways no other Pokemon game has done in quite the same way.
Hoenn really unlocks the adventuring potential of the Pokemon games, and I hope Legends: Arceus does as well. In Gen 3 I scaled volcanoes and fought next to boiling magma, I dived deep under the ocean to catch Pokemon on the seafloor, I explored mountains and sunken ships, vast forests and sparse deserts. Hoenn has a beautiful landscape with loads of nooks and crannies to explore. And it wouldn't be half the region it is without all the water.