Answer:
Game Boy
Step-by-step explanation:
The first handheld game console with interchangeable cartridges is the Milton Bradley Microvariation 1979. Nintendo is credited with popularizing the handheld console concept with the release of the Game Boy in 1989 and continues to dominate the handheld console market.
I personally wasn't alive during the release of the Game Boy, but I have many Game Boy systems today. :)