The most appropriate diagnostic test to perform next is needle aspiration of the left supraclavicular lymph nodes. Given this patient's significant smoking history and his imaging findings consistent with a pulmonary mass and associated lymphadenopathy, he has a high pretest probability of primary lung cancer. The most appropriate diagnostic test is the one that not only confirms the diagnosis but also provides adequate staging to help plan treatment options. Needle aspiration of the left supraclavicular lymph nodes is the least invasive procedure, and it would also stage the likely malignancy by documenting metastatic disease to these lymph nodes.

Bronchoscopy with needle aspiration of the mediastinal lymph nodes would both diagnose and provide some information about staging. However, it is more invasive than aspiration of the palpable supraclavicular lymph nodes.

Bronchoscopy with transbronchial biopsy of the left apical mass may be technically difficult given the apical position of the mass. However, more importantly, it would not provide any information about possible metastases to the lymph nodes.

CT-guided needle biopsy could potentially diagnose the patient with lung cancer. However, the needle would need to pass through normal lung tissue, which carries a risk of inducing pneumothorax. In addition, similar to bronchoscopy with transbronchial biopsy of the left apical mass, it would not provide information on staging because it would not assess whether there is involvement of the mediastinal or supraclavicular lymph nodes.