Let's talk about Meowstic and Nidoking.
Meowstic(F): - C Tier
Availability: Outside of the Parfum Palace. Fairly easy to acquire.
Stats: Average defensive stats, above average SpA and SpE.
Typing: Psychic isn't the great typing that it once was. Team Flare regularly carries dark types which can ruin Meowstic's day, though she does well against the poison types they also carry. She is very fast though, and her varied movepool enables her to set up screens, try to set up Calm Minds on a special attacker, or fire off a quick Thunder Wave. She also evolves fairly quickly at 25.
Movepool: Very nice and balanced movepool. She can learn both screens, Thunder Wave, Shadow Ball, Psychic, Charge Beam, Calm Mind, Energy Ball, Dark Pulse, Thunderbolt, Signal Beam, and Fake Out. This is Meowstic(F)'s saving grace, she has the SpA and SpE to make a variety of attacks work for your team.
Major Battles:
Grant: She should be evolved by now. While she's not bulky, her monster SpA stat should help pump out those Psybeams.
Korrina: Definitely wrecks here.
Ramos: Can wreck Weepinbell. Can also Toxic Gogoat, but he and Jumpluff can hurt.
Clemont: Can set up a Light Screen, but not too terribly effective here. She can learn Dig, though, if you wanted to try to use her.
Valerie: Actually does well here. She can set up on Mr Mime and Sylveon with screens and Toxic and Shadow Ball/Charge Beam away.
Olympia: Same thing as Valerie. Set up and Shadow Ball.
Wulfric: Don't recall using her here.
Malva (Fire): Don't recall using her.
Wikstrom (Steel): Don't recall using her.
Drasna (Dragon): Can beat Dragalge and perhaps Altaria with some setup.
Siebold (Water): Can beat Clawitzer, Starmie, and Barbaracle with Thunderbolt. Might be able to beat Gyarados if it outspeeds.
Diantha: Can beat Hawlucha. Don't recall using it on anything else.
Additional Comments: Mine was named Meowschwitz, and I love it for that. Definitely not a top tier mon, but I enjoyed using it through endgame. Unforunately her lack of bulk and fairly poor in-game typing drop her down to C, in my opinion.
Nidoking: - A Tier
Availability: You can find Nidorino/Nidoran on Route 11. You also get a Moon Stone in Reflection Cave almost immediately.
Stats: Nicely balanced stats. Fairly balanced stats except for its above average Att.
Typing: Poison is an incredible offensive type in game. Adding Ground just breaks it, making it weak to Water, Ice, Psychic and Ground, all of which are pretty easy to spot coming.
Movepool: Extremely varied and useful movepool. Mine at the end had Surf, Ice Beam, Poison Jab, and Earthquake. Nidoking learns Earth Power at 43 and Megahorn at 58, but he learns so many useful TMs, its not even funny.
Major Battles:
Grant: Nidorino should know Double Kick, which isn't the best move, but it does help if you're not packing a fighting type.
Korrina: Resists most of Korrinas attacks and should do well.
Ramos: Does well here with Poison moves.
Clemont: If you have a Nidoking, enjoy digging away. If you have Nidorino still, enjoy digging away.
Valerie: Wrecks. Use a ground move for Mawile.
Olympia: She hurts him.
Wulfric: You should be able to take one ice hit, but this gym might give you trouble.
Malva (Fire): Destroys.
Wikstrom (Steel): Destroys.
Drasna (Dragon): Ice beam ALL THE THINGS!
Siebold (Water): Might be able to do something with Thunderbolt, but they can all hurt him pretty badly.
Diantha: Has the potential to beat Hawlucha and Gourgiest but the others have coverage moves which Nidoking doesnt like.
Additional Comments: Nidoking is better than ever. He can meet whichever role you need him, and he is bulky with a great typing and extremely varied movepool.