The main thing that differentiates different TC's is the accuracy of the machining. The front and rear mounts need to be very parallel. Canon goes to extremes to assure this, and you pay the price. Kenko is also excellent.
Then, there are the plastic body tubes from China which may have flimsy mounts that flex and bend. If you use one rarely and are not hung up on perfection, they are fine. But, if you want the best quality, get Kenko or Canon.
I've had all three, I ended up selling my Canon set, and later, sold my Kenko set. I have a cheap Chinese set that I only use to play with once a year or less, and it serves the purpose.
Now that I am selling many of my light weight primes and getting larger zooms, I may pop for Kenko again. I keep a eye out for used ones on our local Craigslist and have found some deals every few years.