Don’t bother playing the original gen 1/2 unless you do the VC versions out of curiosity. The physical versions of those games cannot carry your mons forward to gen 3+. This is why gen 1&2 had remakes in the first place.

Most gens follow a release like this: OriginalA, OriginalB, Sequel, Remake1, Remake2.

Usually the Sequel game is just an enhanced/fixed version of the OriginalA/B games, so that’s the one you want if that generation has one (e.g. Yellow, Crystal, Emerald, Platinum, Ultra Sun/Moon).

Gen 5’s sequels (Black/White 2) are actually a different story so they stand out as being a little different from the other sequels. Gen 6’s sequel (Z) got canceled and some elements of that (such as Zygarde) seemingly ended up in Sun/Moon instead.

The remake games always have 2 words in the name and are the setting from an older gen (e.g. FireRed/LeafGreen, SoulSilver/HeartGold, OmegaRuby/AlphaSapphire, BrilliantDiamond/ShiningPearl).

These remakes are generally regarded as the best version of that setting. Be aware that they usually introduce mons from later gens in the postgame, so it wouldn’t be a “pure” experience if you want things to be chronological. Also be aware that BD/SP had some negative reviews and some people say it’s worse that just playing Platinum instead since it is missing some features added in Platinum.

I would consider the Let’s Go games to be spin-offs and would only recommend them if you played a lot of Go. They are mechanically different from the normal games, taking several elements from Go.