What is the most effective ingredient in modern chemotherapy?

Depends. It will depend on the type of cancer & the biology of cancer . Different cancer requires different approach and treatment. However i think the answer is a targeted therapy specifically targeted to the mutated receptors in cancer cells. The best story of effective therapy of cancer in modern chemotherapy era is the story of Gleevec (imatinib) for cml.
Targets. In my opinion our ability to tailor meds to bind only to specific proteins, which are hopefully only on cancer cells has made anticancer medicines much more effective and lethal, while lessening the toxicity. Rituxan (rituximab) for lymphoma was the first of these widely available in the us.