For me, also Piranha Creeper Creek. I remember that when I was collecting all the green Stars I had one left in this level, and that was to light up all the fireplaces, it was a pain and it took two hours to do.
Other than that and Champion's Road, none of the levels are painfully hard as far as I can remember.
I'm in agreement on Piranha Creeper Creek After Dark for the Green Stars (Still haven't got the second one), and I'd also add Blast Block Sky Plaza, we still haven't completed it ourselves, a friend of mine came over and completed it, so we don't have all the Green Stars on that one yet.
I concur "Piranha Creeper Creek After Dark", since the second Green Star is difficult to get amidst the darkness, the poisonous swamp, rotating platforms and the Piranha Plants. I am glad to finally got 100%. Plus, I also beaten the final 3 levels (Champion's Road, that Mystery House and the Boss Blitz) with all characters, mainly by bringing as many characters as possible and then activating them at at the goal.