I'll be honest with you, HDN got me with mixed feelings at best. Yes, it has some great gaming in-jokes, but game-wise the system simply isn't THAT attractive.
Ar Tonelico III is shaping up to be at least as fun as its predecessors (I honestly didn't get very far into it yet), which means that's another chunk of my social life I'll lose to stripping songstresses. Disgaea 3 can be pretty fun, though the endgame grinding for specialists (with or without the duping trick) is horribly tedious. (At least they gave you an option so that you can train multiple characters at once by giving you the formation system from Soul Nomad. But still...)
Overall, I suppose I'd say... ATIII > Disgaea 3 > HDN, particularly in the joke department. (And ATIII comes with a side of fanservice if you're not ashamed of enjoying it. There's an actual trophy named "What's wrong with being a pervert?!" in this game. 'Nuff said.)
EDIT: It's all Customs' fault. We have a fucking 47% a year increase in imports and the Guv't doesn't bother with hiring new people, it's fucking frustrating.
EDIT2: If you buy Disgaea 3, DON'T DOWNLOAD THE UPDATE! You'll thank me for it later.