Graphics: A+
Okay, I admit that not everyone will like the visuals, but personally, I love them. The unique "crayon" art style is one of the best I've ever seen. Top notch, indeed.
Audio: A-
The only place where the game isn't perfect. Don't get me wrong, the music is amazing and the game has one of the greatest soundtracks of all time, with cheery, happy songs, dark castle themes, a cool underground song, and a kickass metal song for the final boss battle.. If it was just the music, the audio would have certainly gotten an A+. However, Baby Mario ruins that with his annoying crying whenever you're damaged. To this day, Yoshi's Island is the only game that makes me want to light a baby on fire. It's great, otherwise.
Gameplay/Fun: A+
This game is so much fun! The ability to eat enemies, poop them out as eggs (with a six egg maximum, and throw them is awesome! You also get a flutter jump and a ground pound. There are other cool additions, such as red coins, the ability to transform into different vehicles such as a tank and helicopter, and getting a star and turning Baby Mario into a superhero that can walk over spikes and Yoshi into a giant egg. The minigames, playable by getting a flower (five are in each stage) at the end of the level or unlocking special doors are fun, and the boss battles are great (with the exception of Roger The Potted Ghost, one of the easiest bosses ever), culminating in a final boss battle against a giant Baby Bowser who gets ever closer as you try to hit him in the mouth with giant eggs. The game is challenging, but never unfair. The level design is clever, with one of my favorite stages being Touch Fuzzy, Get Dizzy, in which Yoshi starts tripping balls after touching a dandelion-like puff called a Fuzzy. I have no complaints in the fun department.
Overall: A+
This is one of the greatest games of all time. It's fun and challenging, has a great art style and a fantastic soundtrack, and is my personal favorite Mario platformer, even more so than Super Mario Bros. 3, Super Mario 64, and the original Super Mario World, which is oddly very different from Yoshi's Island, despite the 2 in Super Mario World 2. If it's not the greatest platformer of all time, then it's at least in the top 5, if not top 3. This game is perfect. Well, except for Baby Mario.