I would probably love Yoshi's Island if the game wasn't all that great, simply because it looks so luscious. I can't think of any game that beats it in terms of pure visual appeal. It's 16-bit artistry taken to the limit, from the design to the style to the animation.
Luckily it plays like a dream, is superbly structured and offers enough variety in the levels to hold the interest through to the end. It's also - in true Nintendo style - packed full of secret stuff and things to search for. It's very compulsive to go back into a level time and time again to get 100% on the stars, flowers and coins. In this instance though the collecting isn't tedious because the game is a joy to actually play.