The Nintendo 3DS is the first "Nintendo" video game system that offered demos for both downloadable and retail titles! I figured that it would be fair to separate this article into two sections: One award for the best demo of a downloadable title; and another award for the best demo of a retail title.
The Best Nintendo 3DS Demo of a Downloadable Title
Demos of Downloadable Titles:
Mutant Mudds (Renegade Kid)
Order Up!! (Ignition Entertainment)
Pyramids (Enjoy Gaming)
The Winner: If you're intrigued by the triple layered world of Mutant Mudds, or if you like 2D games in general, you have to play the games demo! The stereoscopic 3D effects are among some of the best on the 3DS!
The Best Nintendo 3DS Demo of a Retail Title
Demos of Retail Titles:
Crush 3D (SEGA)
LEGO Batman 2: DC Super Heroes (Warner Bros. Interactive Entertainment)
Mario and Sonic at the London 2012 Olympic Games (SEGA)
Metal Gear Solid Snake Eater 3D (Konami)
Nintendogs + Cats: Golden Retriever (Nintendo)
Rayman Origins (Ubisoft)
Resident Evil: Revelations (Capcom)
The Amazing Spider-Man (Activision)
The Winner: I enjoyed the Resident Evil: Revelations demo the best. The demo is decent in size, and showcases the games new features (1st and 3rd person perspectives, and Genesis Scanner) well. Here's a video of the entire demo (please note the footage was taken from the Japanese version of the demo):