Slingpod Galaxy, they have a ton of star bits in one area and its easy to die. I managed to 999 in one go.
I finally unlocked Luigi, it sort of bugs me that the luma staying behind has nothing else to it. I'd also like to add that I quite enjoyed the Toy Time Purple Coin challenge, at least far more than Freezeflame, Honeyhive, Sea Slide, Dusty Dune (AUGH) etc.
Still Luigi seemed somewhat pointless and rather unrewarding considering he plays identically to Mario and starts all the way back at level one. Not that I didn't see it coming from this topic, but when she says nothing repeats itself in quite the same way you would have thought that there would have been some variation.
I still can't quite get over the soundtrack for the last level/boss. Considering how much I despise most every other Mario theme (especially underworld) it was a welcome surprise. (Gusty Garden wasn't too bad either)