My vote was for Ouendan 2 but only because of features. Mostly skippng the intro and outro for the songs. Nothing got me more frustrated than watching the 45 second opening for "Ready, Steady, Go" over and over again.
The fact is that I loved them all pretty equally. They're all on my cart, get played regularly, and will probably never leave. Even bad songs became good (or at least fun to play) thanks to those games. Now that I think about it, Ouendan 2 probably had the most songs I didn't like but my vote won't change.
I need me some more sequels.