Just like the topic says. What do you feel is the game you have the most intimate knowledge with? The best understanding? Sheer domination of opponents in said game (if it's multiplayer)? Tell us how good you are/were.
For me, it's Marvel vs. Capcom 2. I definitely can say I put thousands of hours into that game and had reached at least semi-pro competency while it was still a thing. I think my crowning achievements would be one match where I won with only U, UR, R, DR working on the control stick and LK on the buttons using Ryu, Shuma-Gorath, and some third (Spider-man, maybe?)...and a game I played for fun against someone where I took Sentinel/Hulk/Juggernaut and won by turning around and playing with my hands behind my back -- I could look behind me, but I couldn't change hands or turn around. I don't think I could ever do stuff that crazy anymore (at least the hands-behind-the-back thing), but those were some fun times.