What makes a cartridge design iconic? Well, it has to have some character, for starters--i.e., it can't just be a gray hunk of plastic. (I'm looking at you, Mr. Nintendo 64 Cartridge.) Also, it has to have captured at least a portion of the public's attention and imagination during in its lifetime.
In my oh-so-gay-but-not-so-humble opinion, only a handful of cartridge designs (five, actually) have checked both of those boxes and, as such, deserve to be called iconic. Here they are:
Atari 2600--I probably should add an asterisk to this entry, as the colorful labels affixed to Atari 2600 cartridges are chiefly responsible for them earning a spot on this list. Without them, they'd be little more than black rectangles. Taken as a whole, though, the carts are undoubtedly, unquestionably iconic.
Famicom Disk System--Admittedly, Nintendo's shockingly yellow disks aren't all that well known outside of Japan, but that shouldn't keep them from achieving iconic status, should it? Of course, the opposite could be said of the similarly iconic NES cartridge design, below, which is likely to be unknown among the people of Nintendo's home country.
NES--Bow down to what may be the most iconic (among Western gamers, at least) cartridge design of all time. After all, any American (of a certain age) worth his or her salt can identify these gray, ridged and textured hunks of plastic as Nintendo Entertainment System games.
PC Engine--Say what you will about the PC Engine's catalog (or, more likely, the anemic catalog of its criminally ignored American counterpart, the TurboGrafx-16), but you can't fault the sleek, sexy (yes, sexy) cartridges--or, rather, HuCards--that housed the system's games.
GameBoy--Is it just me or do the GameBoy's boxy, gray cartridges look like miniature versions of their NES counterparts? I guess that's why I consider them similarly iconic. Anyway, just between you and me: I prefer the GameBoy cart design thanks to the oval indentation at the top and the larger label space.
See also: 'In my oh-so-gay opinion: The five most iconic controller designs of all time'