Confusion on roll pitch yaw formula

How to determine correct formula for rool, pitch, yaw angle below are the variations I am seeing 

X =Roll axis, Y = Pitch axis, Z = Yaw axis

roll_1 = atan(Y / sqrt(X*X + Z*Z))

roll_2 = atan2(Y,  sqrt(X*X + Z*Z))

roll_3 = atan(Y/Z)

roll_4 = atan2(Y, Z)

Which one is correct and which to use when.

 

 

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