Which of the following studies is BEST in establishing causal association in aetiology of a disease?

Randomized control trials produce best evidence for evidence of causation. But in reality, experimental data may not be available in human beings. Coho studies also provide strong evidence for cause to effect relationship. Crossectional studies provide weaker evidence and ecological studies provide the least satisfactory evidence for cause effect relationship. Ref: Reference: Park 21st edition, page 85; Textbook of community medicine, Sunder Lal, 3rd edition, page 277.