the canons are nice for the white ones starting from 70-200mm but can´t be used with most other lenses.
i think the kenkos give me more flexibility.
Note that the Canon TCs also work with the 135L and 200/2.8L lenses.
I agree the Kenko TCs can offer more flexibility. I'm recommending reliability over flexibility. I'll also point out that you stated:
i want the best image quality.
The general consensus is that while the newest Kenko 1.4x DGX is similar to the Canon MkII, the Canon TCs offer better IQ. Also, with non-tele lenses, the TC generally has a much more significant negative impact on IQ.
I think you need to decide on your priority - flexibility for lenses you may someday buy vs. IQ and known compatibility.