This is a very hard question, I have many different favourites games for different reasons. I think I would have to go with Battlefield Bad Company 2 on this one. The gameplay/teamplay of this game was phenomenal. I was part of a forum and we would squad up to play frequently, so I have lots of fond memories on this game!
Super tricky to decide but I'd have to say The Witcher 3.
No other game ever made has hit the exact right combination of gameplay/story/graphics/immersion/everything.
Everything just comes together so perfectly in that game.
Over twenty years old, but Star Wars: Tie Fighter for me. It was the perfect balance of tactics and strategic gameplay, the mechanics were great, and really immersive for an archaic game with great story too.
Elite starting on my BBC B 32k Computer on 5.25inch floppy. Played the hell out of it and continued playing it through the Years on different evolutions of computer right up to latest modern re-iteration on the PC..
My favourite game of all time would have to be a game that sadly is no longer available. Freelancer. It was a space SIM that could be played both single player or in mmo fashion with a bunch of other players online. You were free to play however you wanted and could amass a fortune by being a bounty hunter or just being a non-violent trader.
Many, many fun hours online with my Dad and my brother's back in the day running around as the "Knights of Freelancer" and just bonding with each other as our first real taste of online play.
My fav game of all time is FIFA (all series), I love the fact that I can make decisions in the game that can't happen in the real sport, plus its dynamic can never get bored, always discover something new.
I grew up with PS1 and PS2 but once I got older I didn't continue playing much untill about 5 years ago. The game that introduced me to the world of pc gaming and got me to join Steam was Left 4 Dead 2. I'm one of those people that doesn't have one single game to name the best or favourite so for that reason Left 4 Dead 2 takes the cake for me.