All 3
Which is the most important to study: Openings Middlegame or Endgame

I think most amateur games today are decided in the middle game, so I would say that. But middle game not so easy to study.
Importance:
Middle game > endgame > opening
The endgame can be studied exactly and its study is the basis for middle game and even opening play.
The middle game cannot really be studied: you can train tactics and you can play games and analyse them and you can analyse grandmaster games.
Opening is largely fashion and opinion.

This is my view ...
Opening - All pieces are protected, literally everyone knows basic things, it's very hard to actually blunder anything. I say this is the easiest phase of the game
Middlegame - Soul of the game, atleast for me every single game was decided in the middlegame potentially
Endgame - Unless your opponent already "knows" what the solution for a particular endgame is, it is just a battle of who has the better tactics.
Therefore, middlegame is the most important atleast for my level

in fact endgames are the most important, knowledge of favorable endgames will lead your middlegame for specific gameplans. Endgames often are clear like water, you know exactly if its win, loss or draw, so without knowing this, you are playing like "lets see what happens". Moreover, as you progress more and more games will be decided in endgames.
on secound place is middlegame, where you should know your strategy (how to play in certain pawn structures, trading bad pieces for good pieces, creating weak squares on enemy part of the board and using them, etc etc.) And being aware of your and opponent tactits.
the last one is opening, its useless to waste hours of opening theory because, under master level, players you play wont follow the theory. From opening you should enter middlegame save, and to know what are common middlegame plans from the opening you play.
in fact endgames are the most important, knowledge of favorable endgames will lead your middlegame for specific gameplans. Endgames often are clear like water, you know exactly if its win, loss or draw, so without knowing this, you are playing like "lets see what happens". Moreover, as you progress more and more games will be decided in endgames.
on secound place is middlegame, where you should know your strategy (how to play in certain pawn structures, trading bad pieces for good pieces, creating weak squares on enemy part of the board and using them, etc etc.) And being aware of your and opponent tactits.
the last one is opening, its useless to waste hours of opening theory because, under master level, players you play wont follow the theory. From opening you should enter middlegame save, and to know what are common middlegame plans from the opening you play.
Funnily enough my list is the opposite of this atleast for my level.
First up, i disagree that endgames are the most important. I think reaching to a level where you are well versed with endgames and "know" the plans requires many years of dedicated work and anyone under pro level isn't very well versed in this. For most people endgames are just tactical problems.
Endgames, for most people, are the least important.
Opening is way more important than people think even though i think they are much easier than middlegames. In almost all of my games people play the opening way too much by "feel", as in they just look at a move and if it looks good they play it. They don't calculate through different variations and don't spend much time and that's why i have had a few games where i had some advantage out of the opening and i don't know any theory at all. It's just that thinking minute details of things in an opening is very underrated.
Middlegames are the most important imo since it's full of tactics.
Just my list though, found it funny that it was basically the reverse of yours xD
Which stage of a normal chess game do YOU think is the most important to study.
endgame by far. The opening is the least important just need to know opening principles. Most fo the GM's will tell you that opening doesn't matter. Like Finegold, ChessCoach.net, Chessdojo., etc... And when you look up chessbooks, you will see its mostly endgame books that are written and mostly endgame books that are recommended. Like Jeremy Silmans Endgame Manual for beginners and intermediate, or Dvoretsky for experts.
Opening is underated, endgames are overrated

Depends on level.
Up to about 700 or 800 you can win a lot of games with a sharp opening against opponents who don't know how to defend pieces and positions or how to develop. After that, most players know how to develop without getting into too much trouble. The middle game tactics become more important at that point and it seems most games get decided in the middle game for a while. I'm starting to get to the point at 1050-1100 where a significant number of games are going to the end game - that's becoming increasingly important and definitely the weakest part of my game and something I'll need to work on if I want to get to 1200+. I'm already struggling the most with end games.
I would imagine after 1200 to 1500 - everything becomes important as players become good enough to capitalize on small advantages.

- Depends on you. If you are a beginner who knows nothing, then I'd just suggest you to get introduced to all the opening and their patterns. Study the masters' games related to those openings. Learning the patterns is the key concept here. Learn basic principles and common traps. In this way, you will be able to cover openings and middle game together.
Memorize checkmating patterns to the core. It’s very important.
If you are familiar with most of the openings, then focus on two pet opening systems and study them deeply and master them.
Then, slowly approach to tactics training and what not. After that, you can approach to endgame.
However, once you become really good at endgame, you will learn that openings are important but not THAT important where you need to spend so much time on them UNLESS you are a Master or competing with masters. You will be able to understand positions far more deeper than ever. You can study endgames before middle games. It honestly depends on your style.
Which stage of a normal chess game do YOU think is the most important to study.