Best is a difficult thing to quantify unless someone has gone to the effort to try all the variants.
I recently purchased a 70-200Mk2 and Canon 2x III, and already had a Canon 1.4x II. With only a small amount of testing (a couple of days shooting passenger aircraft) I see no major difference between the two during the test. Yes sharpness takes a very slight hit, and the percentage of sharp shots decreases with either TC. The question comes down to whether the trade off is worth it. For each individual will have their own idea of what is acceptable. Do you have the option to purchase a TC, test it for a few days and return it if not satisfied?
70-200mk2 + 2x TC III...
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