A few of them worth looking into:
Nidoqueen

Just barely outclassed by the male counterpart, and an example hardcore Feminazis can use to describe Male Privilege, Nidoqueen is a Pokemon that one should look out for. Here's why:
- Access to 3 HMs, including Surf (!) and Strength
 
- Well rounded stats
 
- Excellent movepool, if you include TMs. Otherwise, you're going to have to a wait a long while to get what you want.
 
- Few key resistances.
 
- Easy to evolve, if you have a Moon Stone (also a con)
 
- Found at a low-ish level ingame, at 21, so it is, I believe, quite early ingame.
 
Cons:
- No stat is really you know, excellent, as such. However, it doesn't matter so much  ingame.
 
- Typing gives it a weakness to some of the most common types of attacks faced, such as Ground, and Ice.
 
- Requires a Moon Stone.
 
- Nidoking looks cooler 
 
Scolipede

Not quite under-used I know, but it is often ignored in-game.
Pros:
- Excellent Attack and Speed
 
- Great Hidden Ability
 
- Useful in competitive as a Baton Passer
 
- Again, found at a low level, so I'm assuming it's early in-game.
 
- Works well against the Fairy-gym
 
Cons:
- Predictable movepool, but this is fine, to be honest.
 
- Earlier stages need support and minor amounts of grinding
 
- Doesn't look 'cool'. I mean, most Bugs don't, with the exception of Beedrill and a few others.
 
Crobat

 An old favourite of so many, but often ignored because Zubat is annoying, and Happiness to evolve makes it a pain sometimes.
Pros:
- Excellent Speed
 
- Decent, well rounded stats.
 
- Effective movepool (Again, somewhat predictable, but that's okay.)
 
- Found EVERYWHERE (Almost  a con, tbh), as in, most other games, if you want to transfer
 
- Found at low levels, quite early ingame.
 
- Can learn Fly
 
Cons:
- Needs happiness to evolve, but Poke-Amie, and other things have somewhat fixed that.
 
- Needs TMs to function better
 
Skuntank

Pretty much hated by all thanks to how annoying it was in the Sinnoh games, and that owned-by-the-baddies vibe, but it can be pretty fun.
Pros:
- Great HP and Speed
 
- Decent attacking stats that work well in-game, but not in competitive
 
- Good in competitive as an Annoyer, or something on those lines (Taunt + Sucker Punch + Explosion)
 
- Decent number of TMs that compliment stats
 
- Mediocre mixed sweeper. (Learns Flamethrower for some reason)
 
- Excellent typing that gives only 1 weakness and 4 resistances and 1 immunity.
 
- Found at low levels early-ish in game.
 
Cons:
- Evolves at level 34.
 
- Poor Defenses, but quite okay, to be honest.
 
Other mentions: Beedrill, Nidoking, Dragalge, Drapion and Gengar, and Venausaur (OU, but fantastic).