There's a page on Bulbapedia all about talking pokemon. Excluding the episodes that starred Pokemon only and the ones that possessed Meowth to speak through him, there's Meowth, Mewtwo, Slowking, this Rotom now, a Gastly that one time in Gen I. Entei from the Unknown movie, Arceus from Jewel of Life, Lucario from Mystery of Mew, Zoroark sometimes, Hoopa, Santa's Lapras (no joke) used telepathy in the banned Jynx episode, Ash's Larvitar spoke in a dream world, Darkrai during his debut movie, Manaphy because it learned some words like a baby, and Chatot which is just a parrot.

Additionally, lots of Pokemon used telepathy to speak human, though usually it was during the movies. The telepathetic ones include Diancie, Carbinks, Xerneas, Volcanion, Genesects, Zekrom, Reshiram, Zorua, Lugia, Ralts once during the Gen III anime, Shaymin, Jirachi, Kyurem, Keldeo, Virizion, Cobalion, Terrakion, and the Zygarde cores.