Least helpful for the diagnosis of spondylolisthesis -
All the given radiological investigations are used for spondylolisthesis. However, AP x-ray of spine is least helpful as in low grades of spondylolisthesis it may be normal.
On the other hand lateral x-ray, CT and MRI (sagittal view) diagnose even the mildest degree of spondylolisthesis.
It is worth noting that spondylolysis (not spondylolisthesis) may be missed on lateral x-ray and MRI. CT scanning is more sensitive in diagnosing spondylolysis.
AP x-ray of spine
Lateral x-ray of spine
MRI
CT