The best replacement for fiddler is wireshark.

The best alternatives to fiddler are rated as following:
★★★★★  wireshark
★★★★★  charles
★★★★  firebug
★★★  violinist
★★★  httpwatch
★★  chrome developer tools
★★  chara
★  burp suite
★  postman
★  http analyzer
  tcpmon
  hermit

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

Other search terms related to fiddler and their ratings are:
★★★★★ soapui
★★★ windows phone emulator
★ slotomania
You have in mind a companion for fiddler and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of fiddler in all years. Click on graph for more.
fiddler
wireshark
charles