The best replacement for fms is dcs.

The best alternatives to fms are rated as following:
★★★★★  dcs
★★★★  iim
★★★★  xlri
★★★★  red5
★★★★  wowza
★★★  sfma
★★★  airfield
★★★  parkzone
★★★  fmf
★★  dynam
★★  durafly
★★  isb
★★  iim kozhikode
★★  cfs
★★  ces
★  iim lucknow
★  nasm ces
★  mdi
★  dbms
★  clearview
★  iift
★  rc desk pilot
★  iim indore
  direct commercial sale
  jbims

Your alternative for fms is not listed? Suggest!
Speaking about fms, one probably shouldn't forget about dx6i.

Other search terms related to fms and their ratings are:
★★★★★ dx6i
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Google Trends

Scale is based on the average worldwide traffic of fms in all years. Click on graph for more.
fms
dcs
iim