In a middle position, if you make a positional error that will make your endgame chances of winning pretty low, if you make a tactical mistake you may as well say goodbye to the endgame and the game itself.
Opening is important because trust me.. if you start out bad and have a cramped position throughout the whole game, you will probably have no winning chances and very few drawing chances,
i would say endgame is the least to worry about--- but VERY important to study.
Just my 2 cent.
If I may interject....
I believe that the most important 'phase' of the game, is when and wherver an important tactical situation comes up. Recognising that there is tactical play at hand is extremely important as, wether it is to your advantage or against you, the proper implementation of the tactic's mechanism or its counteraction (respectively) will be the 'game makers' or 'game savers' or if you fail to recognise them (and your opponent does not) then the 'game breakers'.
If you look over most good games, free of major blunders, then you will see that time and time again, a good tactic was responsible for the winning advantage.
Also it is almost taken for granted at some point that there will be no major opening errors or endgame blunders, so having said this, and factoring in what was already said along with the known rich tactical opportunities it has - I would have to say the the mid-game offers the most important opportunities overall (i.e. tactical ones) which makes it the overall most important phase of the game.
In addition, consider the fact that endgames tend to be more simple to play due to the restricted possibilities, whereas opening play is much more predictable due to the restricted decent or reasonable possibilities. As such, these two phases are played more rapidly, or with overall greater ease. It is always during the midgame that people slow down and ponder more carefully, intuitively (and analytically) realizing the critical nature of the at times delicate, intricate and/or tempo-sensitive situations that arise there.
Otherwise, you can't have an endgame without a midgame, and no midgame without suriving the opening, so the rest is academic...
Well the endgame is the most important technically since the entire game leads up to a winning, drawing or losing endgame.
As for which phase is most important to be skilled at, well that's a different question.
I think that the middle game is the most important, because of setting up for the endgame. The middle game is also my favorite, so another reason. However, it is important to set up a good position in the opening, and be succesful in the middle game to set up a good endgame. Another way to look at it: most wins come towards the endgame, so therefore, it is important. For me, I'll go with a strong middle game, FILLED with tactics. 
The endgame is the most important. My style of play is to analyze pawn structure, then swap pieces down in favourable exchanges. If I somehow grasp a material lead going into an endgame the other player is in trouble.
There are two important points.
The transition from opening to middlegame and then from middlegame to endgame.
All 3 phases of chess are important. If you mess up in the opening you are not likely to make it to an ending. The same applies to the middlegame, mess up there and get mated or lose a piece to prevent mate and you are lost. If you are losing in roughly equal endings then by all means study endings. If you are already lost by the time you reach an ending then you need to study that phase in which you lose more often. I never studied endings much ( except for basic endings ) until I was consistently reaching roughly equal endings and being outplayed, then it was time to work on endings.....
Okay, here's another question, if you were to study one of the three, then which? If you memorize openings then you are totally clueless on what to do in the middlegame or endgame, and all you really do is memorize and not really learn chess. The middlegame is important, but there's less room to maneuver your pieces, and too many to choose from. In the endgame if you were for instance had a bishop and some pawns and your opponent had a knight and some pawns, then you're focused on the bishop. Don't forget also, the king. In the middlegame you need to keep your king safe, but in the endgame you use it and realize how important this awesome warrior is.
Some people say that memorizing the openings is the most important, others say the endgame, snd very few say the middlegame. I'll get straight to the point, I think it's the endgame because in the opening or middlegame, you make the slightest mistake and you're still in it, but in the endgame that can turn a winning postion into a draw or loss. Plus, memorizing openings doesn't improve your game, but by studying the endgame you experience the real potential of the pieces.