TL;DR:
To be honest, I have to say it's probably Counter-Strike: GO / CS2 – there's no other game I've invested so much time into over the years and still continue to play.
Followed by DayZ – the summer of 2012, when the mod came out and kept evolving week by week – we were a group of more than 10 people who were online every day and basically lived in the game. It was just something completely different from anything else I had experienced in multiplayer gaming up to that point.
Other than that, I have wonderful memories of split-screen games like Mario Kart 64, Halo CE, Resident Evil 5, Gears of War, Army of Two, Kane & Lynch – but that time won't come back, and I won't be able to gather those old friends again to relive it.