The best replacement for coax is optical.

The best alternatives to coax are rated as following:
★★★★★  optical
★★★★  hdmi
★★★★  rca
★★★  cat5
★★  toslink
★★  ethernet
★★  s-video
★★  fiber
★★  fiber optic
★  twisted pair
★  digital optical
★  optical audio
★  component speakers
★  cat6
★  cat 5
★  optical better
★  ethernet fios
  bnc
  rca better
  triax
  s video
  utp
  rg59 coax
  hdmi cable
  ladder line
  spdif
  digital coax
  rg6 coax
  ethernet uverse
  glasvezel
  component cable
  cat5e
  optical cable

Your alternative for coax is not listed? Suggest!
Speaking about coax, one probably shouldn't forget about direct tv.

Other search terms related to coax and their ratings are:
★★★★★ direct tv
You have in mind a companion for coax and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of coax in all years. Click on graph for more.
coax
optical
hdmi