The best replacement for x11 is x31.

The best alternatives to x11 are rated as following:
★★★★★  x31
★★★★★  xquartz
★★★★★  x12
★★★  px21
★★★  x41
★★★  terminal
★★★  dx11
★★★  vnc
★★★  xorg
★★★  wayland
★★  xlc
★★  framebuffer
★★  x10
★★  mir
★  p11
★  z2
★  x10 klipsch
★  rf1000
★  x1
★  triton ax180
★  ax 180
  tritton trigger
  xl1
  gtk
  xfree86
  directfb
  honda x4

Your alternative for x11 is not listed? Suggest!
Speaking about x11, one probably shouldn't forget about cygwin.

Other search terms related to x11 and their ratings are:
★★★★★ cygwin
★★★★★ putty
★★★★ sudo
★★★★ ssh
★★★ window manager
★ dss2
★ mixamp
You have in mind a companion for x11 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of x11 in all years. Click on graph for more.
x11
x31
xquartz