The best replacement for windbg is ollydbg.

The best alternatives to windbg are rated as following:
★★★★★  ollydbg
★★★★  softice
★★★  cdb
★★★  debugdiag
★★★  gdb
★★  adplus
★★  visual studio debugger
★  ida pro
  ida
  mdbg

Your alternative for windbg is not listed? Suggest!
Speaking about windbg, one probably shouldn't forget about remote debugging.

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



Google Trends

Scale is based on the average worldwide traffic of windbg in all years. Click on graph for more.
windbg
ollydbg
softice