#1 Chex Quest
PC: 1996
In Chex Quest, you—a human piece of Chex—are tasked with rescuing your delicious brethren from the planet Bazoik. You see, the first wave of Chex pioneers has since been imprisoned by the "Flemoids," a particularly nasty alien race of slime people.
Apart from being among the only videogames to feature actual phlegm as an enemy, Chex Quest holds the distinction of being the first-ever prize-in-a-cereal-box game. Certainly there have been other "advergames"—Tooth Protectors, Cool Spot, Avoid the Noid, Kool-Aid Man—but those games appeared either on store shelves or in the mailbox.
Here's what makes Chex Quest truly notable, though: it's Doom. Like, it's seriously just Doom, here re-skinned for younger players. (Every instance of in-game "blood" has been turned from red to an inoffensive green, so that blood now resembles "slime." Tricky!)
Although there are only two official sequels to Chex Quest, fan-made Doom mods are myriad; even Wolfenstein has been given the Chex treatment. And, in a horrifying reversal, here's what Wolfenstein looks like on planet Bazoik.