What is the most critical part about chess?

Tactics? Endgame study? Create a list!

You gotta survive the opening to begin with or else you don't get to anything else (tactics, endgame etc)

Making the first move is the most critical part.

Really.

If you make the first move without the intentions of playing the best you can, then you will find yourself playing unsatisfying games.

The other items; openings, middlegame, tactics and engames are all secondary to this one thing.

 Visualization.

Agreed.  In doing 1600+ problems on Tactics Trainer, I realized that the ability to visualize the board 4 to 5moves down the line is critical.  I'm sure at a higher level, one will need to be able visualize much much further down the line.