Surprise: it's the best 2D Mario game.

Yoshi's Island has some of the absolute best level design in any game I've played. Yoshi's controls are a real highlight, making for a lot of flexibility in your platforming and projectile-tossing. The game also excels aesthetically, with a very appealing and cohesive visual style. Honestly, its visuals are one of the game's major improvements over Super Mario World, in my opinion.

The levels having no timer, combined with being able to recover Baby Mario if you get hit, encourage you to take your time exploring the large and well-designed worlds. The scoring system nudges you this way as well. If you want to just head straight to the exit, you can, but I found that this game is actually a lot of fun to explore to reach 100% completion.

One of the very best Nintendo games, period.