Wario Land: Super Mario Land 3

This is the first game where Wario is playable. Mario's speed and jump-height are traded for a horizontal dash and attack. Collecting money and hunting secrets are meant to be important. Consequently, linear levels are replaced with rooms that you backtrack through; it fells like Kirby level design. The worlds and opponents are original, yet they visually and conceptually fit Mario's universe. Wario Land is frequently difficult, and it punishes you hard for mistakes. That difficulty, combined with a less intuitively-fun character, frustrated me enough to drop the game. Maybe it is less of a pain if you skip collectibles and side-content.