The posterior urethra is best visualized by -
Voiding cystourethrography is the best method to visualize posterior urethra.
Remember,
Urethra can be imaged radiographically in two ways. 

Anterograde techniques → Best for visualization of posterior urethra.

(This is done along with voiding cystourethrography or with voiding following excretory urography)

Retrograde technique → Best for examining the anterior (penile) urethra 

​(Contrast is injected through tip of urethra).
Static cystogram
Retrograde urethrogram
Voiding cystogram
CT cystogram