I am going to go with Lanturn. It has good move coverage with ice, electric, and water. It usually gets overlooked a lot since people either start with totodile or they use the shiny gyarados as their water type. So it tends to not get to shine and show what it can do.

The grass type starter in basically every generation. Everyone always picks the water type or the fire type. There's never any love for Bulbasaur, Chikorita, Treecko, Turtwig, etc.

Going generation by generation

Gen 1 - Poliwrath! This guy is my favorite gen 1 Pokemon and an absolute beast. He has a very solid stat distribution which allows him to be naturally tanky, and his typing is incredibly useful. He kind of gets overshadowed by the many other water types in Kanto like Blastoise, Gyarados, Lapras, ...

Gen 2 - Forretress. Steel typing is obviously great, but almost everyone looks to the Scizors and Skarmory's of this world. Forretress shares Scizor's incredible typing and is a great utility pokemon, getting entry hazards, thunder wave etc. He's not just a passive wall either, he's got a very respectable attack stat and can hit surprisingly hard with Gyro Ball, Bug Bite, Payback (almost always boosted to 100 due to his slow speed).

Gen 3 - Ninjask. Everyone knows how broken Shedinja is, but Ninjask is incredible too! Speed boost is obviously a great ability, but pair that with swords dance and baton pass and you have a pokemon that can turn any team member into a sweeper. Bug/Flying is on paper not an incredible type, but honestly I found plenty opportunities to safely set up a swords dance, and from there you don't even need Ninjask anymore as you baton pass into your other pokemon. Great support pokemon!

Gen 4 - Drapion has incredible stats and an even better typing. It is bulky, shockingly fast and you can run a very fun sniper set with a scope lens item and moves like Night Slash and Cross Poison to score many destructive critical hits.

Gen 5 - Archeops often gets dismissed because of its ability, but honestly this guy is a main stay on my Unova teams and I swear the ability has never let this thing down. It just destroys everything in its path before Defeatist can even activate. Those STAB acrobatics just sweep through entire enemy teams before you can blink.

Gen 6 - Avalugg unfortunately has one of the worst types in the game. Nevertheless is is still an incredibly underrated pokemon. The sheer physical bulk it has makes anything not super effective essentially just a tiny scratch, and it hits quite hard back.

Gen 7 - Araquanid has one of the most broken abilities and this alone turns it into an underrated beast. Sticky web on top of that? Wow

Gen 8 - Runerigus was a monster on my team. Great typing and stats, and the ability can make you disrupt enemy pokemon that rely on their abilities (huge power pokemon for example). Probably the stupidest evolution method ever though so nobody uses this thing.

Gen 9 - Lokix. What?? The route 2 bug type is a big threat? Yes it is, it hits hard as hell and gets crazy moves including first impression.

I'll say the Pokemon that was the hardest for me to catch because until recently it hadn't been in a game in a long time and that's Purugly it was the last Pokemon I caught to complete my pokedex before Gen 9 added more Pokemon also as a bonus it's fat like my cat Sweetpea.

Bulbasaur, he's my favorite Pokémon yet nobody knows he exsist.

Literally everyone knows who Bulbasaur is, what? He's one of the most iconic Pokemon.

Anyway, I'll throw Pinsir in here. In gen 1 it was like a pairing with Scyther, but I feel like Scyther got so much more love between the two of them, getting two whole evolutions. Pinsir was meanwhile kind of replaced by Heracross in the Johto games. They did give it a mega evolution but it must be one of the most forgotten ones.

I used it recently in a rom hack and really enjoyed it! The Mega is very strong and fun to use.

Bulbasaur?! No, the most underrated Pokemon is Pikachu!!

On a real note, since everyone else has probably named the actual most underrated Pokemon, I think I'm gonna go a bit more of a silly one (but I do genuinely mean underrated!) and go with Furfrou. I watched my best friend do an XY nuzlocke and that thing, unbelievably, was the GOAT for the entire run from the moment he was in the party. XY are not known for being particularly difficult games but my friend was having comically bad luck in pretty much every fight imaginable and my man(dog?) was a brick wall you could lean on because no matter how bad the luck was, he'd still tank it.