The most underrated Pokemon imo would have to be Sawsbuck which is an amazing deer Pokemon. Sawsbuck is a lot of fun to have on your team and it's a Pokemon that doesn't get talked about a lot and nobody seems to use on their team, mine is a level 78 right now with 239 HP, 184 attack, 136 defense, 133 special attack, 157 special defense, and 178 speed and has Solar Beam (120 power), Double Edge (120 power) High Jump Kick (100 power), and Nature Power (- power) it's not the greatest stat wise but it is an awesome Pokemon and one of a few Pokemon that stay on my team at all times I also like the whole seasons thing (although it doesn't seem like it changes seasons after you've caught it)

I wouldn't say it's neccessairly the most underrated but from the top of my head Porygon2 comes to mind. Give it an eviolite and it's a pretty great all around Pokemon. Has a useful scouting ability in trace, gains great bulk, pretty decent special attack and while it's not particularly fast it's not the slowest Pokemon either. It also has access to a very wide movepool both offensively like STAB tri attack, ice beam and thunderbolt and defensively with stuff like recover and thunder wave meaning it has lots of different options in how to build it. Which can even make it a better option than its evolution sometimes. I also love its design and concept. Definitely one of my favorite all around Pokemon.

Aside from that paraflinch Pokemon, namely Dunsparce and Togekiss. You'd be suprised how effective of a strategy it can be both ingame and even competetively. Dunsparce is great for trolling people with if you can set it up it first, and thanks to Togekiss pretty good stats and decent coverage it can be just an all around force to be reckoned with. I remember on one of my first playthroughs of Pokemon SoulSilver I more or less defeated the entire elite four only using a level 47 Togekiss. I think I only used two hyper potions throughout. 

I feel as though both Snorlax and Gliscor are two underrated pokemon. Sure they both have a stat total of over 500, and they both have OU potential if they are set up correctly, but no one talks about them nearly as much, or in the same way that people talk about Charizard and Mewtwo for instance. 

Gliscor is great because it has an excellent typing that makes it immune to electric type moves, coupled with its excellent poison heal ability (which allows it to receive passive recovery while being poisoned through the use of a poison orb). Gliscor also has access to roost, swords dance, knock off, and earthquake. With proper EV investment, you can craft yourself an effective stall breaker. 

The same goes for snorlax, if you set it up with a curse set and train it in the correct EVs, you can set it up as an effective sweeper using return as its main STAB move, and rest to recover HP. 

Ultimately, there are other great pokemon out there that have very usable moves, stats, and abilities, but they just don't get the same recognition and praise as some of the more well known pokemon that hog the limelight.

I'd have to say 44hy, not many people use the glitch pokemon because its only in the first generation and its only useful in its evolved version, Q, which is used to fuse pokemon into hybrids. I wish people would like it a little more, since its kind of cute for a mess of pixels and it has a cool name.

Honestly Glalie (more specifically Mega Glalie).

It hits extremely hard with refrigerate STAB return / double-edge and can learn a few other moves that also really help it (crunch, earthquake, ...)

And there's also the infamous explosion tactic, which destroys anything that doesn't resist it and has decent defense. Time it right and you can catch your opponent on a switch and get rid of all their momentum.