Ophie
Salingerian Phony
Kingler is definitely underrated. Even with its new GMax form I still don’t see it being used very often or even talked about past the first two months after SwSh were released. Venonat and Venomoth also seem to be underrated. They don’t even appear in any Pokédex that doesn’t have you being in Kanto at some point in the game. Every other Gen I Pokemon appear in at least one additional Dex that is outside of the Kanto and Johto region. Even the Legendaries and Mew.
I've been long awaiting for Venomoth to return in some region's native Pokédex. They tried real hard to give it more attention by some then-unique, nifty Abilities like Tinted Lens and Wonder Skin, but they've all since been used by other Pokémon as well (notably Sigilyph, who is similarly not talked about much).
As for Kingler, I've always felt it got potential ever since the physical/special split in Generation IV. That being said, I think this is a Pokémon that will eventually get its day, even if it might not even be in this generation. Tentacruel was long dismissed as fodder until Generation V when it was discovered that this thing has a lot of resistances, very nice Special Defense, Rapid Spin, and Liquid Ooze to discourage Leech Seed, and it's been hanging on since, even if Toxapex stole its thunder in a big way.
Generations 5 through 8 have a LOT of underrated Pokemon that really do need some more love. Seismitoad, Lilligant, Galvantula, Florges, Pyroar, Gogoat, Clawitzer, Wishiwashi, Lurantis, Boltund, Sandaconda and Copperajah just to name a handful. As far as pre-Gen 5, I think Lanturn is massively underrated, I've heard a lot of people call it average or even down right weak, but every time I've had Lanturn on a playthrough team it's been an MVP and does really well.
Lanturn came up a couple of times in the Armor Beginnings competition; I had a surprisingly hard time with them. That beig said, I only saw them among Japanese players. Those guys really know how to think outside the box when it comes to making teams.
(By the way, Seismitoad has since become a staple of rain teams.)
I do agree with the rest of those, but Pyroar, for some reason, gets Abilities that boost Attack despite its Attack not being that great (which is odd considering the animal it's based on) but a much better Special Attack. I get the hunch Pyroar will see a lot more use if, say, Unnerve or Moxie is replaced with Competitive...which might not really happen, not only because Pyroar already has three Abilities, but lions in real life are not competitive creatures. (I think I'll go put a more in-depth explanation on that in the "Did You Know?" topic.)