Klefki isn't very common now, but honestly, it should be. It has great typing, and while it's stats aren't the best, it has extensive moves to corner your opponent. Klefki's base HP is terrible, but Recycle and a pinch berry (Wiki, Aguav, Mago, etc.) should keep it in longer than expected. It can also be obtained quite easily by going to Thrifty Megamart. I definitely recommend checking Klefki out.
Both of these Pokémon have terrible stats, so it shouldn't be on your team. But they do have Tail Glow, which raises its Sp. Atk by 3 stages. Not much to say. Probably don't use this Pokémon.
Liepard has an amazing Speed stat, but it has underwhelming Attack and Sp. Atk, and frankly, horrible Defense and Sp. Def. It won't last long.
Meowstic is actually a really good option. It can act as extremely good support with moves like Light Screen/Reflect, Fake Out, Helping Hand, and so on. It's a fast Pokémon with mostly decent stats and a varied movepool. Just avoid equipping it with Sucker Punch and other physical moves. Meowstics are more of a special attacker, so yeah.
Overall, I'd recommend Klefki over Meowstic because you need to trade for Meowstic. You could trade from the GTS or ask a friend though. One of them isn't really better than the other, so it's up to whose moveset aids your team more.
Also, you should consider other moves that slow down speed if you haven't already, like Icy Wind and Bulldoze.
I hope this helps :D!