The only real hard stage is surfing in the trial galaxy.

the giant monty mole boss

i havnt played all stars, but i thought the trial galaxies were hard, and so was the purple coin star at the boos haunted mansion place

Yeah I thought the garbage star was pretty easy =P But then again, there is a "trick" to it. 

 We all know the trick...hopefully.

The only star that ever gives me trouble every time is the Sea Slide Cosmic Luigi race. This is because Cosmic Luigi misses only 2 shortcuts in the whole race. The rest I can tend to get within 1 to 5 tries.

Trial Rolling Ball.

Woah woah woah! Slow down there turbo. There is a trick?

The Purple Coin Stars. But man, they were so fun!

Luigi's purple coins, without a doubt. The only time I got a game over.

Put the bombs on the coin giving shooty points.