A prequel to the iconic Kingdom Hearts games, Kingdom Hearts Birth by Sleep (2010) is a well-written introduction to the Kingdom Hearts universe without requiring people to play other Kingdom Hearts games.
The game showed many revelations that were previously unanswered in Kingdom Hearts (2002) and Kingdom Hearts II (2005) including Xehanort (Leonard Nimoy) who was actually a Keyblade Master.
The game had an all-star voice cast including Jason Dohring, Jesse McCartney, Willa Holland, Mark Hamill, James Woods and Haley Joel Osment (Not Sora, obviously).
The game had three storylines, each based on the characters Terra, Ventus and Aqua. It is best to play the game with those storylines in that order because it makes much more sense.
Birth by Sleep is what a prequel video game should be made.