Before I go any further, there is one reason why you need to play "Yoshi's Island".  During one of the boss battles, Mario and Yoshi are fighting a giant frog's Uvala while trapped in the frog's stomach (it's a weird battle, to say the least); after defeating the frog, the frog then literally expels Yoshi and Mario out through its rectum.  Without a doubt this is the strangest scene in the entire Mario series, even with the "Peach is Bowser Jr's mother" scene in "Super Mario Sunshine.