The original KQ2. KQ2+ gave the game, well, a reason for being there, what with the actual plot and all, and making it much more relevant to the rest of the series. The original? Bah! You walked into the tower room and this girl says she'll marry you because you gave her some earrings you found in a tree and paused to ask her name?! Not *quite* my idea of finding true love. Valanice had more involvement in KQ2+, though, and it actually gave her a reason for liking Graham, too.
I'd otherwise vote for KQ7, I think. While I enjyoed it and all, it seemed very... separate from the other games. Oh, sure, tied in Edgar again and all, but... I dunno. The others maybe connected more for me.