Counter-Strike 2.0 (CS 2.0) is a version of Counter-Strike 1.6 (CS 1.6) with better textures, models, sounds and sprites. This version comes with different graphics in order to keep up with the rival Counter-Strike: Global Offensive. Of course, there is a difference in engines, but I think there was a good job in GoldSrc for CS 1.6.
Counter-Strike 1.6 (CS 1.6) is one of the mos populare FPS ( First Person Shooter ) game which was released in 1999 and played by many people. Even today it is a good game and still played, not like it used to be, but still. A lot o players started playing CSGO, because it has better graphics and new system, but I think CS 1.6 is better because of it simplicity.