I totally made this topic like two months ago! Giving the nod to World now as I did then, and as I likely always will. Tighter level design of 3 can't surpass the sheer scope of World (plus Yoshi).
Although 64 remains the best Mario title.
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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.