A patient presents with a solitary pulmonary nodule (SPN) on X-ray. The best investigation to come to diagnosis would be –
"Thoracotomy with resection represents the most direct and definitive means of diagnosis (and treatment) of solitary pulmonary nodule"."Thoracotomy is the most definitive method for determining the diagnosis in patients with SNPs".

However, a thoracotomy is associated with significant morbidity, complications and mortality. Therefore, CT guided a transthoracic biopsy is the invasive diagnostic procedure of choice.

Note -If the question would have been about the next investigation of choice (instead of best investigation), then the answer will be CT scan.
MRI
CT Scan
USG
Image guided biopsy