Opening, Middle Game, End Game

What do you guys think is the most important to study in trying to develop into a stronger chess player?

My opinion is openings, especially traps (games under 20 moves) since it seems like 90% of beginners fall for known opening traps over and over again.  So study short games to see what to play or what to avoid.  It helps you plan ahead and you must recognize a trap and ask yourself, what is he treatening or what am I threatening.  Once you have mastered a few openings, go on to tactics or middlegame.  Endgames are for the masters and above.  But masters will tell you to study the endgames first.  Right.  So they can beat you in the opening and middlegame.  Maybe less than 5% of my games have ever gone into some sort of endgame that was tough and hard to figure out and that I should have studied.  But those games I could have done better in the opening or middlegame.  So play lots of games so you can get the pattern recognition of what works or not.  Whatever the most important to study, have fun.  Learn from your mistakes.  Try to get someone better than you to analyze your games and see where you went wrong.  There are at least a dozen bad moves in almost every amateur game.  Your job is to get it down to just a few bad moves.  Play what opening you feel comfortable with.  Any strong player can play any opening and win.  Find one that works for you, but be familiar with the traps, both White and Black, in that particular opening.

For beginners, the middlegame, more specifically Tactics, are very important to learn. After tactics, Endgames are, to me, more important than openings.

Note, this is after learning basic opening principles and basic endgames, along with castling and other such beginning chess knowledge.

Openings~ mastering a variety of openings will make you a dangerous player~

Middle game. But you can't become strong if you only learn one part of the game.

If the key word in the OP is "study" then I'd say middle game. Familiarity with opening principles will serve one well, and the number of even end games reached will be relatively few until the "stronger player" stage is reached.

Best to study all phases though.

Neither, they are all equal parts.  The end-game is where you learn to 'play Chess'.  Or so the legend goes.  But if your opening or middle game is bad your not likely to see many endgames.