My coolest cart would be the Vs. Super Mario Bros. Reproduction cartridge I got a few years back from retrousb.com. It is the arcade version of the game which is different than the NES version. The cart is crystal clear and you can see the rom board. Plus it has the dip switches to set the amount of lives, continues, timer speed, and coins for a 1-Up. Sadly they discontinued the Vs. Reproduction carts. I hope they start doing so again.
Right on. VS [shoot the dog] Duck Hunt is my personal fav though. The ramped up difficulty also makes it feel not so much like a kiddie game. Once I got to level 38 on Clay Shooting with the original NES port and game play quickly got boring after that. The VS game alternates Ducks on the odd rounds; Clay pigeons on the even rounds. On Normal and Extreme difficulty setting, rounds 5, 6, 11, and 12 you have three ducks to shoot with only three bullets!
Any NES homebrew / repro game in a translucent shell from RetroUSB or elsewhere. Tengen black releases with their large print labels. Color Dreams / Wisdom Tree blue shells. Gold Zeldas.
Kirby Tilt 'n' Tumble translucent pink Game Boy Color cartridge.
Men-A-Vision Air Raid. No I don't own one, but they look freakking awesome!
The rainbow of Famicom carts if you're into imports.
Also if you count the cool packaging, any Famicom Mini series import GBA game with the package within a package, also series 1-20 with two-toned white / burgundy carts. I have the SMB2j game. The Disk releases (21-30) are in solid yellow however, and contain an Easter Egg that lets you watch the FDS demo by holding A+B on startup.
Atlantean PCe/Turbo homebrew.