The best replacement for kenko is hoya.

The best alternatives to kenko are rated as following:
★★★★★  hoya
★★★  nikon teleconverter
★★  canon teleconverter
★★  canon extension tubes
★★  kewpie
★  tamron teleconverter
★  hoya filters
★  sigma teleconverter
★  canon extension
★  hoya uv filter
★  canon extender
★  tamron
★  hoya filter
★  marumi
★  sekonic
★  nikon extension tubes
  tiffen
  hoya pro1
  canon tubes
  opteka
  meike extension tube
  hoya uv

Your alternative for kenko is not listed? Suggest!
Speaking about kenko, one probably shouldn't forget about sigma.

Other search terms related to kenko and their ratings are:
★★★★★ sigma
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kenko
hoya
nikon teleconverter