For reference, the type combinations with one weakness (and what that weakness is) are Normal (Fighting), Electric (Ground), Normal/Ghost (Dark), Water/Ground (Grass), Poison/Dark (Ground), Bug/Steel (Fire), and Ghost/Dark (Fairy).
/nds normal, monotype (75 results)
Aipom, Ambipom, Arceus, Audino, Bidoof, Blissey, Bouffalant, Buneary, Bunnelby, Castform, Chansey, Cinccino, Delcatty, Ditto, Dubwool, Dudunsparce, Dunsparce, Eevee, Exploud, Furfrou, Furret, Glameow, Greedent, Gumshoos, Happiny, Herdier, Kangaskhan, Kecleon, Komala, Lechonk, Lickilicky, Lickitung, Lillipup, Linoone, Lopunny, Loudred, Maushold, Meowth, Miltank, Minccino, Munchlax, Oinkologne, Patrat, Persian, Porygon, Porygon-Z, Porygon2, Purugly, Raticate, Rattata, Regigigas, Sentret, Silvally, Skitty, Skwovet, Slaking, Slakoth, Smeargle, Snorlax, Spinda, Stantler, Stoutland, Tandemaus, Tauros, Teddiursa, Terapagos, Type: Null, Ursaring, Vigoroth, Watchog, Whismur, Wooloo, Yungoos, Zangoose, Zigzagoon
/nds electric, monotype (37 results)
Ampharos, Arceus-Electric, Bellibolt, Blitzle, Boltund, Eelektrik*, Eelektross*, Electabuzz, Electivire, Electrike, Electrode, Elekid, Flaaffy, Jolteon, Luxio, Luxray, Manectric, Mareep, Minun, Pachirisu, Pawmi, Pichu, Pikachu, Pincurchin, Plusle, Raichu, Raikou, Regieleki, Shinx, Silvally-Electric, Tadbulb, Tynamo*, Voltorb, Xurkitree, Yamper, Zebstrika, Zeraora
/nds normal, ghost:
Zoroark-Hisui, Zorua-Hisui
/nds water, ground:
Barboach, Gastrodon, Marshtomp, Palpitoad, Quagsire, Seismitoad, Swampert, Whiscash, Wooper
/nds poison, dark:
Drapion, Grimer-Alola, Muk-Alola, Overqwil, Qwilfish-Hisui, Skuntank, Stunky
/nds bug, steel:
Durant, Escavalier, Forretress, Genesect, Scizor, Wormadam-Trash
/nds ghost, dark:
Sableye, Spiritomb
There are also a handful of Pokémon that have one weakness thanks to abilities. These include Levitate Koffing and Weezing (Psychic), Levitate Rotom Wash (Grass), and Lightning Rod Goldeen and Seaking (Grass). Additionally, thanks to Desolate Land, Primal Groudon is only weak to Ground.
*Eelektross and its pre-evolutions have no weaknesses thanks to Levitate.
Prior to the Fairy type's introduction in Gen 6, Ghost/Dark types had no weaknesses.
