I have the 1.4 and 2 TCs series III. I bought the Kenko DG 300 Pro 1.4 so I could focus at f/8. Unfortunately, it locked up my 5DIII with some lenses and I had to remove the battery to restart. So, I sold it. If you can afford it, get the series III for long term compatibility with newer Canon lenses. They are already essential for the series II telephoto lenses.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M 18-55, f/2 22, 11-22