Clanker's Cavern and Wet-Dry world are awesome, if only because they made me deathly afraid of going deep underwater. Such a trip to play those levels as a kid and finally get the star/jiggy after so many failed attempts!
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http://www.youtube.com/watch?v=SBpfwpOPXNc&NR=1&feature=fvwp
I watch Planet Chasers Starlight Excellent all day, every day.