I enjoy the entire series, it's hard to pick one favourite! I'd probably be tempted to pick ghouls n ghosts just because it seems like the most balanced of the games - definitely the easiest in my opinion plus the special weapon needed to clear the second run of the game is actually decent, compared to the almost useless cross in the original gng.
That said, the Kai release of the psp game is amazing as well, so much better than the original psp version of he game. It does an excellent job of rebalancing the difficulty plus gets rid of the terrible gold rings fetch quest the original game required to get e best ending, now you just need to clear the game twice as per all previous gng games.