I'm pretty biased while reviewing certain games, but I'd have to say I'm especially biased with Yoshi's Island. You see, Yoshi's Island was the first SNES game I played, at the tender age of five years old. My family was pretty late in the game (heh) when it came to the SNES; we were a Sega Genesis family throughout the early-mid 90's. My dad bought a SNES in 1997, so we ended up the the SNS-101 model (which I still use as my main SNES to this day, screw the haters), and I suspect he thought it was a new console... but I'm not entirely sure. Later that year, or perhaps early the next year, we got a Sony Playstation. If you read the linked Wiki article, you'd know that Yoshi's Island was one of the pack-ins, and was indeed the one we got with it.
Firstly, it must be stated that Yoshi's Island is pretty divergent from the Mario titles that came before it, in many ways: