These are the powers to use on the blocks that seal the rainbow drops in their rooms.

World 1 Grassland Use Parasol Power
World 2 Forest Use Spike Power
World 3 Ripple Field Use Rock Power
World 4 Iceland Use Fire Power
World 5 Red Canyon Use Spark Power
World 6 Cloudy Park Use Cutter Power
World 7 Castle Dedede Use Ice Power

To get the last rainbow drop (in the last level), get and use the powers in the following order:

Fire
Stone
Spike
Ice
Cutter
After going through the last door, you will be below the drop.