Bladder cancer can occur in those who are working in chimney for 25 years. Which is the best study for this relationship ?

As a single study unit, double blind RCT is the best: "The efficacy of new interventions are most readily accepted if the results are from randomized controled trial" Peer Review & Biomedical publication However, overall meta-analysis is a better study since it combines the data from multiple RCT and also from other types of study. "Randomized controlled trials (RCT) provide the stongest, most relevant evidence to inform practice. Some evidence hierarchies place systematic review and meta-analysis above RCTs since these often combine data from multiple RCTs, and possibly from other study type as well" - Epidemiology at a glance Ref: Park 24th edition