depends on which version of counterstrike you want.
There is the original counterstrike which you can download for free if you own Half-Life, there is Counterstrike Condition Zero which as I understand it is just the original Counterstrike with better graphics, and then there is Counterstrike Source which is yet another remake of the original but with much better graphics.
I've only played Source but people say that the original counterstrike is better in all areas except the graphics.
Condition Zero is hardly ever talked about or played and I have no idea why. It seems that it has all the good things from the original (which Source lacks) and it has decent graphics (which the original lacks).
I'd go with Source or the original because those seem to be far more popular than Condition Zero (why everyone plays the original and not Condition Zero I have know idea) and you will surely find many more servers for them.