OoT and MM are two of the best games ever created in mine and many others on this forums' opinion. The aspects that make these masterpieces stand out are near uncountable.
I think the biggest reason these are games are regarded as legends in this series is because of how big of a jump they were from the previous four games. While these first four games were remarkable, the graphics were mediocre (except I thought LttP's were amazing). The N64 games were the first 3D Zelda games, and might I add they were an aesthetic masterpiece. The game's visuals were extremely awesome, and they introduced a whole new way to play Zelda. These 3D graphics allowed for a whole new fighting system. It was no longer just "hack at the moblin with your sword until they die". First of all, you can lock on. Battling is way easier and feels much more natural when you're locked on. Then there's the ability to slash different ways. Jump and vertical slash, stab, spin attack, and everything else. And you can also side step, back flip, crouch with your shield up, etc. 3D fighting overall gave a much better experience to the Zelda game.
The 3D Hyrule that OoT provides gives us much more freedom, and the same goes for Termina in MM. For a lot of the previous games, there weren't many towns/significant regions, and there wasn't much other than the dungeons. Because of this, the stories were much deeper and easier to fully understand, and there were many sidequests/subplots. OoT had plenty of sidequests to get things such as the Biggoron Sword and Ice arrows, but MM really went haywire with the sidequests and I loved it. If I counted right, you only need 4 of the 20 non-transformation masks. The remaining 16 provide a sidequest of some degree, and sometimes the sidequests were very extensive and quite a blast to complete. The premiere one is the Couples Mask, which requires us to create the well-known Anju and Kafei sidequests, which requires us to do various specific tasks and specific times throughout the three days. But the overworld isn't the only good part; the dungeons were greatly improved. Instead of a normal 2D plane with all of the same puzzles, the 3D dungeons have much more complex puzzles and structures. Instead of having random puzzles, the dungeons have more of a "main theme", such as the gears and water flow in MM's Great Bay Temple, and the Dark and somewhat scary atmosphere of OoT's Shadow Temple.
The items in these games also introduced a great aspect. Because of the capabilities of the new game engine, items were much more fun to use. In the previous games, every item was very simple, and just going in one direction and pressing a button. However, in these two games, the items are much more complex. You aim in first person with the bow and shoot in a very specific place. The Great Fairy items gave very interesting abilities. The ocarina has you play specific songs and plays a huge part in the storyline. Overall, every item is more fun to use, and lead to even more interesting items in future games such as TP.
I feel that these games really made a huge leap in the Zelda series to introduce tons of new aspects, and kind of laid out the map that future Zelda games have followed. Because of the impact that these games made, they are considered the best games in the Zelda series and sometimes the best games of all time.
I think the biggest reason these are games are regarded as legends in this series is because of how big of a jump they were from the previous four games. While these first four games were remarkable, the graphics were mediocre (except I thought LttP's were amazing). The N64 games were the first 3D Zelda games, and might I add they were an aesthetic masterpiece. The game's visuals were extremely awesome, and they introduced a whole new way to play Zelda. These 3D graphics allowed for a whole new fighting system. It was no longer just "hack at the moblin with your sword until they die". First of all, you can lock on. Battling is way easier and feels much more natural when you're locked on. Then there's the ability to slash different ways. Jump and vertical slash, stab, spin attack, and everything else. And you can also side step, back flip, crouch with your shield up, etc. 3D fighting overall gave a much better experience to the Zelda game.
The 3D Hyrule that OoT provides gives us much more freedom, and the same goes for Termina in MM. For a lot of the previous games, there weren't many towns/significant regions, and there wasn't much other than the dungeons. Because of this, the stories were much deeper and easier to fully understand, and there were many sidequests/subplots. OoT had plenty of sidequests to get things such as the Biggoron Sword and Ice arrows, but MM really went haywire with the sidequests and I loved it. If I counted right, you only need 4 of the 20 non-transformation masks. The remaining 16 provide a sidequest of some degree, and sometimes the sidequests were very extensive and quite a blast to complete. The premiere one is the Couples Mask, which requires us to create the well-known Anju and Kafei sidequests, which requires us to do various specific tasks and specific times throughout the three days. But the overworld isn't the only good part; the dungeons were greatly improved. Instead of a normal 2D plane with all of the same puzzles, the 3D dungeons have much more complex puzzles and structures. Instead of having random puzzles, the dungeons have more of a "main theme", such as the gears and water flow in MM's Great Bay Temple, and the Dark and somewhat scary atmosphere of OoT's Shadow Temple.
The items in these games also introduced a great aspect. Because of the capabilities of the new game engine, items were much more fun to use. In the previous games, every item was very simple, and just going in one direction and pressing a button. However, in these two games, the items are much more complex. You aim in first person with the bow and shoot in a very specific place. The Great Fairy items gave very interesting abilities. The ocarina has you play specific songs and plays a huge part in the storyline. Overall, every item is more fun to use, and lead to even more interesting items in future games such as TP.
I feel that these games really made a huge leap in the Zelda series to introduce tons of new aspects, and kind of laid out the map that future Zelda games have followed. Because of the impact that these games made, they are considered the best games in the Zelda series and sometimes the best games of all time.