Which is the commonest abdominal mass in neonate :
The commonest intra - abdominal tumor in first two years of life → Neuroblastoma.
The commonest intra - abdominal tumor between 2nd to 5th  year of life → Wilm’s tumor.
The commonest intro - abdominal tumor in children (no age specification) → Neuroblastoma.

(Wilm's tumor is the second most common abdominal tumor in children).
Remember: 
The commonest cause of abdominal mass in Newborn is Multiple dysplastic kidneys (if neoplasm or tumor has not been mentioned when asking for the commonest intra-abdominal mass, the answer will be multiple dysplastic kidneys).
Wilm's tumor
Polycystic kidney
Neuroblastoma
Rhabdomyosarcoma