With first generation of Pokemon we were presented with 151 unique monsters. Many of them become popular among fans because of their "bad-ass" look and powerful attacks. There are probably no one who played that GameBoy game or watched Pokemon TV series who do not remember pokemons like Charlizard, Arcanine, Gyarados, Gengar or Dragonite. But among them were also those pokemons that look week and lame, but in fact are really powerful. Here's most underrated party (6 pokemons) of 1st generation (in random order):
Psyduck / Golduck - this psychic duck has bad reputation because of anime. Mistys Psyduck was a dumb bird that jumped out of pekeball in random moments and could not lay a single attack. In fact Psyducks are quite interesting, as a mix of water-psychic type they are able to use attacks like Confusion, Hydropump, Blizzard, Ice Beam which in combination with high special attack (especially after evolution into Golduck) is perfect weapon against Dragon, Ground, Rock, Toxic and Fighting pokemons.
Mr. Mime - looks like a weirdo clown or old geezer. But has special attack at similar level as Dragonite and Arcanine. Using TM we could teach him to use Thunder, Solar Beam and Psychic which makes him another great chose against water types, dragons, fighting types or rock pokemon.
Lickitung - yes it may look stupid with that tongue and Team R Lickitung were really weak, but its a real one-man band. Variety of attacks that Lickitung can learn is enormous: Surf, Mega Kick, Mega Punch, Thunderbolt, Thunder, Earthquake, Seismic Toss even Fire Blast and Blizzard, super effective against any type you can think about including Dragons.
Jigglypuff / Wigglytuff - same as above. We could now it as singing cutie that draws on peoples faces, but it actually could learn whole bunch of electric, fire, ice, psychic, grass or fighting moves, and Wigglytuff has special attack as high as Blastoise and Cloyster.
Clefairy / Clefable - another one-man band like Lickitung and Jigglypuff but with special attack on even higher level.
Rattata / Raticate - nightmare of Pokemon GO players, and also one of most common pokemons in GameBoy editions. You could find it everywhere so most people think of it seriously but except normal attacks it learns during training we could teach it some extra moves like Dig, Thunder or Blizzard. It may not have highest stats but its easy to train and you could easily get as many of them as you want.