Which Pokémon can speak with humans?

In the Pokémon anime we have the ability to create human speech with their mouths (as opposed to telepathy, possession, etc):

Pokémon anime

Without any special circumstances

• Team Rocket's Meowth (the vast majority of episodes)
• Gastly (EP020 The Ghost of Maiden's Peak)
• Chatot (multiple episodes)
  • Repeats human phrases it has heard
• Rotom Pokédex (multiple episodes in Sun & Moon)

With special circumstances

• Larvitar (EP263 Address Unown)
  • Only while in the Unown dimension
• Zoroark (BW127 The Island of Illusions!)
  • Only after transforming itself into Nurse Joy
• Mirror-world Meowth (XY037 The Cave of Mirrors!)
• Diggersby, Pangoro, Delphox, Froakie, Chesnaught (XY098 Dream a Little Dream from Me!)
  • While in a dream created by Darkrai

Films related to the Pokémon anime

• Slowking (Pokémon the Movie 2000: The Power of One)
• Entei (Pokémon 3: The Movie - Spell of the Unown: Entei)
  • In the English dub, an echo filter is added to suggest telepathy, but that doesn't exist in the Japanese version
• Manaphy (Pokémon Ranger and the Temple of the Sea)
  • Only able to say simple phrases like "happy" and "I love you"
• Darkrai (Pokémon: The Rise of Darkrai)
• Arceus (Pokémon: Arceus and the Jewel of Life)
• Hoopa (Pokémon the Movie: Hoopa and the Clash of Ages)

And in Pokémon the Movie: I Choose You!

In general, any Legendary/Mythical Pokémon in one of these movies will either have human speech or telepathy.

Pikachu shorts

• Snover (Pikachu's Ice Adventure, only aired in Japan)

Video games

• Generation I (Red, Blue, Yellow) and Generation III remakes
  • Copycat's Doduo, which repeats human speech
  • Bill (as hybrid Pokemon)
  • Marowhak's Ghost
• Generation II (Gold, Silver Crystal) and Generation IV remakes
  • Murkrow owned by Team Rocket, which says a password
• Generation IV (Diamond, Pearl, Platinum)
  • Chatot, by imitating human speech through the Chatter move
• Black 2 and White 2
  • Zoroark, disguised as a backpacker
  • Smeargle (Pokéstar Studios "Love and Battle" series)
  • Ledian (Pokéstar Studios "Timegate Traveler" series)
• Generation VII (Sun, Moon, Ultra Sun, Ultra Moon)
  • Rotom (when inside the Rotom Pokédex)
  • Mimikyu
  • Oranguru (says "Go du it!")
• Detective Pikachu
  • The titular Detective Pikachu

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Source: Talking Pokémon

The only Pokemon I know of which can speak human speech are:

• Meowth
• Ghastly
• Slowking
• Entei
• Darkai
• Arceus
• Snowver
• Hoopa
• Lucario
• and the Rotom Pokedex

All of my information was from https://bulbapedia.bulbagarden.net/wiki/Talking_Pok%C3%A9mon