Super Mario Land: 1989 launch title for the Game Boy and the weakest traditional Mario game in retrospect. You aren't going to find many players claiming this to be the best in the series, even though it was influential in the success of Nintendo's new portable endeavor.
The primitive pixel art doesn't reflect the true graphical capabilities of the Game Boy, as demonstrated by later titles such as SML's own follow-up, Super Mario Land 2. With that in mind, artist Arne Niklas Jansson redid all the sprites and created a patch to apply them to that SML ROM you happen to be stashing on your hard drive. The changes are more subtle than in your typical graphical update, but they are still quite lovely.
Compare the before and after shots in the gallery below.