Ocarina of Time is the benchmark because it is the most widely acclaimed game in the series. The game also happens to be commonly considered the greatest game ever made (https://en.wikipedia.org/wiki/List_of_video_games_considered_the_best#List). I once did a quick and limited analysis of game reviews of the various Zelda games for times other games in series are mentioned. No surprise I found that Ocarina of Time is mentioned probably about five times as often as the rest of the games in the series combined (don't quote me on that because I did this a while ago and don't remember the exact numbers).
Ocarina of Time will continue to be the benchmark until it is clearly surpassed. However, considering that the game has perhaps the best supported claim to being the greatest game ever made, even if the game is outdone by a later title, Ocarina of Time will remain a benchmark to some degree. To surpass Ocarina of Time, a game would have to be of excellent quality, be revolutionary to some extent, and capture the attention of people to a greater degree than Ocarina did. In my opinion every 3d Zelda game has been of excellent quality, and they have been revolutionary to varying degrees (arguably Skyward Sword is the most revolutionary imo). However, none of the games have commanded as much attention, critical acclaim, or cultural relevance as Ocarina of Time.
Ocarina of Time will continue to be the benchmark until it is clearly surpassed. However, considering that the game has perhaps the best supported claim to being the greatest game ever made, even if the game is outdone by a later title, Ocarina of Time will remain a benchmark to some degree. To surpass Ocarina of Time, a game would have to be of excellent quality, be revolutionary to some extent, and capture the attention of people to a greater degree than Ocarina did. In my opinion every 3d Zelda game has been of excellent quality, and they have been revolutionary to varying degrees (arguably Skyward Sword is the most revolutionary imo). However, none of the games have commanded as much attention, critical acclaim, or cultural relevance as Ocarina of Time.