For:
Dragon Quest Heroes: Rocket Slime
The colourful adventure and humor always manages to make me enjoy each second of my playthrough.
Dragon Quest Monsters: Joker
Very well done graphics
Ouendan 2
I felt the song selection was better than the first one.
Against:
Pokemon Diamond/Pearl
This is only essential for competitive battlers, in terms of single-player, this is the worst Pokemon game I have played so far, I was literally struggling to keep myself interested until after I beat the elite four when I can finally get into the competitive aspect of the game.