My choice will always be Counter Strike, because it is the game with which many people shared and learned strategy, and one of the first games I played online with people I did not know in the world, for that sentimental value I choose.
I played CS 1.3 and it's part of my childhood, I still play CSGO but I'm more playing Dota 2.
CS and Dota are both part of my childhood and I went into competitive scene with Dota but no luck with Dota 2.