Viva Pinata was pretty underplayed. It's a really chill, simple garden/animal raising thing on the 360, so it's understandable that it didn't get a lot of play considering it was meant to be accessible to children and the general marketing push for the console seemed to be on shooter games like...
I still need to rewatch a few old favorites but my current favorite has to be Hidamari MOTHERFUCKING Sketch. It's like pure joy condensed into audio/visual form.