After analyzing the board, column 5 is my best move. 

First, I checked if I had any immediate winning moves. I don't see any opportunity to connect four O's in any direction with a single move.

Next, I checked if my opponent (X) has any threats I need to block:
- In column 5, if I don't place my piece there, my opponent could play there next turn and create a horizontal line of X's in row 1 (X X X X). I need to block this immediate threat.

Additionally, playing in column 5 gives me some offensive potential:
- It creates a diagonal (ascending) alignment with my existing O's in the bottom row
- It doesn't set up any obvious winning opportunities for my opponent in the row above

Other columns are either less urgent to defend or might create opportunities for my opponent. Column 5 is both defensive and has some offensive potential.