LogFAQs > #963182970

disgaea 1 has the best plot and is pretty simple, if you don't like its odd pacing and terrible balance you can consider the second game
disgaea 2 is a bit more complex than the original and irons out a lot of the problems while still being relatively simple, definitely a good starting point if you're annoyed by the first game's jank
disgaea 3 and disgaea 4 are convoluted messes of games that add so many obnoxious barriers that might be annoying to deal with, making once-simple things big pains to deal with while not adding much real complexity or depth with a lot of the new additions (it's amazing how the eviilty system ends up being incredibly boring)
disgaea d2 is a return to simplicity like the original two but it's also the only direct sequel; it's my favorite of the ps2/ps3 games due to its pacing and balance even if the story sucks and it was put together with toothpicks and glue