Emerald (the improved version of Ruby and Sapphire) is still early Pokemon and gives you that adventure feel of having to ration your money and stock up on items before the long and arduous trek to the next town, bushwhacking through the marshes during a thunderstorm. The relief of finally making it to the town on the other side before all of your Pokemon fainted. Sailing across the vast ocean on the back of your Pokemon, getting lost in labyrinths of coral reef and island cave mazes. Beware that this game was meant to be viewed on an unlit GBA screen, so it will not look quite right if you emulate it on a bright monitor.
Black & White is the most difficult (not overly so, you do need to be prepared and the final stretch can be a little troublesome) mainline game due to the strength of the trainers and your more curated options. It also has a solid story, the best of the mainline games, and it does so without constantly trapping the player in long unskippable cutscenes every time you walk forward five feet like in Gen 6 onwards. Unova is also the last region that feels like its own world rather than a straight up copypaste of IRL France or Hawaii like Gen 6 onwards. Probably my only complaint is that it is here that the adventure aspect begins to evaporate, as the region is a straight line to the next town. There is no interconnected world like in the prior games.
Pokemon Omega Ruby/Alpha Sapphire is a remake of the Gen 3 Ruby/Sapphire. ORAS is not replacement for the original RSE so I would hesitate to recommend playing ORAS instead of RSE, but it might still be worth playing if you are really into Pokemon. It dilutes the adventure aspect by constantly healing you and teleporting you around, giving you a team wide EXP share that makes the game easy, desaturates and washes out the colors, etc. However, ORAS gives you easy access to a massive roster of Gen 4 and 5 Pokemon that were added after RSE, including prior legendaries like Entei and Reshiram. You also get Mega Evolutions, including MEs for non-Gen 1 Pokemon (X&Y only had MEs for Gen 1). You still have to buy X&Y if you want to get Kalos Pokemon since ORAS does not include those. If you don't want to play all of the games to acquire your favorite Pokemon, then ORAS is an effective time saver, though transferring Pokemon can be a little difficult nowadays with the closure of the 3DS store. Unless you already had the Pokemon Bank installed, you might have to jailbreak your 3DS to get Pokemon Bank onto it if you want to transfer Pokemon from Gen 5 games onwards. If you are emulating then AFAIK you can't practically transfer Pokemon, so ORAS loses a lot of its value IMO.
Sun & Moon was the last mainline game I played and was the point where I jumped off the train. The positives are that the game has some fun stuff like the trials, and ofcourse some of the new Pokemon designs are interesting. The not-Hawaii/tropical vacation aesthetic is generally pleasant. However, there are many issues. By this point, the challenge of adventure has completely evaporated. There is no difficulty getting from point A to point B. You never have to worry about running out of items or money or not being powerful enough to beat a trainer. You can pretty mindlessly spam your most powerful moves over and over and do not really need to consider doing something different. The setting also feels like our modern safe world, rather than a unique vast and untamed fantasy world full of possibility like in early Pokemon. Mega Evolutions are technically still here, but you cannot transfer your ME stones with your Pokemon to this game, and the ME stones are very difficult to acquire (most of them were limited time event giveaways), so unless you use a cheat device you effectively lose any Mega Evolutions that you transferred over, and there is no endgame content to play with your old Pokemon. I regret transferring my Pokemon over, since it was irreversible. Also, you get constantly trapped in unskippable cutscenes, and you aren't even the main character of the story. You are an accessory to someone else.
After S&M, the compilation of issues combined with the announcement of Dexit for SWSH (basically half of the roster would not be present in that game, meaning you can't transfer a lot of your Pokemon over) along with the general decline of game design and aesthetics and customer respect led S&M to be my last purchase.
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Pokemon Stadium 2 for the N64 exists if you just want to pick your favorite pokemon designs without having to progress through an RPG campaign, run around hunting for Pokemon, catching them and levelling them, etc. You just pick and get straight into 3D battles. The grandiose stadiums, the camera angles, the animations, and the voiced narrator helps elevate the experience, making battle a little more engaging to watch than the actual mainline games (even the later 3D ones). It also has some neat minigames.
There is also
Battle Revolution for the Wii, same thing with the narrator and the camera angles. The stadiums are not quite as large and grandiose. You get access to some Gen 3 and Gen 4 designs. The main issue however is that you do not get a near unlimited roster to pick from like with Stadium 1 & 2. You instead only get a handful of Pokemon you can pick. If you want more then you have to import them from your Gen 4 DS cartridge.
Pokemon Colosseum for the Gamecube has the stadium series camera angles and animations for battle. However, it is a campaign RPG game. So you don't just boot up the game and pick your favorites and go like with Stadium or Battle Revolution. In Colosseum, you play as a defector from evil, setting out to set things right and defeat his former comrades. It takes place in a vast, barren desert with no wild Pokemon to catch or farm exp on, so your availability of Pokemon is limited. The game is tuned around this, so it is the most challenging Pokemon game I have played. Colosseum is also a 2v2 format for the whole game, which is much more interesting than the default 1v1 for the rest of the series.
Pokemon Mystery Dungeon: Explorers of Sky is set in a different world. It is a Diablo/roguelite game where you play as a Pokemon adventurer and his 3 teammates going through randomly generated dungeons. It is tied with Black & White as having the best story in the series.
There are other games worth playing like Snap for the N64, Hey You Pikachu, etc, but they would not be my first recommendation.
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