EF 1.4x 11 or III on 70-200L
I have the Mk. 1 version my understanding is that optically its identical to the Mk. II version - the Mk. II version added weather sealing only. The Mk. III is especially optimized for the new super telephoto lenses but the general opinion is that for older lenses there's not much advantage.
On the contrary the 2x Mk. II is re-designed optically and does out perform the Mk. I