The best replacement for counter strike is counter strike source.

The best alternatives to counter strike are rated as following:
★★★★★  counter strike source
★★★★  crossfire
★★★★  halo
★★★  call of duty
★★  parabellum
★★  combat arms
★★  tf2
★  insurgency
★  half life
★  source
★  team fortress
★  sudden attack
★  battlefield 2
★  battlefield
★  team fortress 2
  cod4
  condition zero
  america's army
  quake 3
  unreal tournament
  soldier front
  rainbow six
  modern warfare

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Speaking about counter strike, one probably shouldn't forget about steam.

Other search terms related to counter strike and their ratings are:
★★★★★ steam
★★ hamachi
★ steam download
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counter strike
counter strike source
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