The Legend of Zelda (NES)
These are the reasons why it is the best game of all time.
1. Best NES game - Not only is it my favorite NES game (and just my plain favorite game), but it was consistently voted that way throughout the NES era. Before the internet came around, the common narrative was that Zelda 1 was the best game ever made. Here are a few quotes from Zelda 1's wikipedia entry: "it was the first NES game to sell over 1 million", "The game placed first in the player's poll "Top 30" in Nintendo Power's first issue and continued to dominate the list into the early 1990s." "The Legend of Zelda was also voted by Nintendo Power readers as the "Best Challenge" in the Nintendo Power Awards '88. The magazine also listed it as the best Nintendo Entertainment System video game ever created, stating that it was fun despite its age." "The Legend of Zelda is often featured in lists of games considered the greatest or most influential. It placed first in Game Informer's list of the "Top 100 Games of All Time" and "The Top 200 Games of All Time" (in 2001 and 2009 respectively)...."
2. It Killed Off the Arcades - Throughout the 70's and early 80's it was understood that the arcade games were always superior to the home games. Super Mario Bros. showed that the home versions could be equally good. But The Legend of Zelda showed that you could have a game that was better than the arcades. The action in Zelda 1 was just as intense as any arcade game, but it also had the depth, exploration and character development of a PC RPG. Because Zelda was better than anything you could play in the arcade, this game, more than any other, contributed to the death of the arcades in North America.
3. Most Innovative Game - Zelda 1 is arguably the most innovative game ever made. You have to realize, first of all, that Super Mario 1 was considered an extremely innovative game in it's day. Think about that. Just the fact that the screen scrolled in 1 direction and there was music playing and a second button for running and fire -- all of that was extremely new at the time. However The Legend of Zelda and Super Mario Bros were developed at the exact same time. Zelda had 4 way directional exploration. It had dual purpose items that could be used for both combat and to find secrets. Hell, it had far more secrets than any game that came before. It was the perfect marriage between arcade action and PC depth. It was the first console game to actually use a save state. It was also the first major console game to use anything like hit points. It was so freakin' innovative, that they included a hint guide with the game. This is also why you don't start with a sword in Zelda 1. Miyamoto wanted people to realize that this game was not like anything that had come before.
4. Ages Like Wine - Some old games still are fun decades later and some are not. Zelda 1 is the only game I've played that was amazing the first time and somehow got better with later replays. This is because Zelda 1 has so much freedom, while at the same time your choices are extremely important. For example you can choose the order in which you complete the dungeons. Also you can choose to do a wooden sword only run or even a swordless run (up until the last boss). There are a lot of ways to play this game and each one makes the game feel different. I once chose dungeon 3 first and got the ladder without having the boomerang or bow and arrow. The game felt totally different. And yet it made me realize that the ladder is also a combat upgrade. You can hover over water and attack foes which makes you more powerful in combat. This leads to my next point.
5. Many Items Have a Dual Purpose - Many items upgrade your combat ability while allowing you to explore the game farther. I've already mentioned the ladder, but the candle and bomb are two other obvious items. The items are not just there to unlock the next area like it a lot of other games. The items almost always make you more powerful. Zelda 1 is an actual RPG. You become more powerful as you progress through the game. Even the items you might think of being about puzzles and secrets upgrade your combat ability too. This leads us too...
6. Never Been Outdone - Super Mario Bros 3 is clearly better than Super Mario 1, and Final Fantasy 3, 5 and Tactics are clearly better than the first. For these two franchises the later iterations felt "the same yet better". Zelda 1, however, has never had a game that was "the same yet better". Zelda 2 is lacking in exploration. A Link to the Past is too easy and lacks a lot of the freedom of Zelda 1. Breath of the Wild actually does have that level of freedom, but neither the dungeons or combat are as good as the original. And the other 3D Zeldas...instead of making a better game, they simply made a very different kind of game. It is very common for a sequel to be a "same yet better" game, and yet Zelda 1 has never been bested at its own way of doing things.
7. Second Quest - After completing this amazing and highly polished game, what do you get? A whole another quest to complete. You basically get a whole second game. Today's games will sell you DLC to download. This is like a major DLC pack that is also free and included on the physical copy of the game. It is far more generous than anything the modern game industry will offer you. In fact, Nintendo is not this generous anymore. Imagine if you'd completed Mario Galaxy 1 and then found you'd unlocked Mario Galaxy 2 and got it for free already included on the disk. That is what the second quest in Zelda 1 is like.
For all of these reasons and more, The Legend of Zelda (NES) is the greatest game ever made, Nintendo or otherwise.