Dragon Quest, known as Dragon Warrior in the US, is a Japanese-style role-playing game created by Chunsoft for the NES and released in 1986.
It's the first game of the famous Dragon Quest/Dragon Warrior series that reached chapter 11 in 2017, not counting the spin-offs.
The game represents a real milestone in the history of console RPGs. Before that, the genre acquired popularity thanks to the home computers, with the Ultima and Wizardry series, which clearly inspired Chunsoft. Dragon Quest is probably the founding father of the so-called JRPG genre.
Due to the platform's limits, the world is relatively small: only 5 cities and 5 dungeons; besides, there are only 10 spells available to the player, and the inventory is limited. But at the same time, exploration is completely non-linear (unlike, for example, The Legend of Zelda), and your limits are only the difficulty of the monsters.
Dragon Quest's popularity is not limited to video games. Especially in Japan, it's part of pop culture and a true social phenomenon. Outside Japan, it's a bit less popular, but if you are passionate about the history of video games, you cannot miss it.