Counter Strike 1.6 ( which i'm still playing every day) and Condition Zero ( the classic of CS actually)
Note: It's much better to use OpiWiki's items -> ranking to handle such questions. The ranking has been created: Best Valve games of all time.
VALVe is an American video game development and digital distribution company headquartered in Bellevue, Washington, United States. More: opi.wiki.