Metal Gear Online 2. I found a clan early on, and played with them consistently nearly every night for 4 years. Weekly survival matches and tournaments. We were a well oiled machine. I will forever look back on that time as some of my favorite online gaming memories. TEMP will forever live on.
Also, playing GTAO ON PS3 and PS4 with fellow era/gaf members. We would spend all our time stunting off everything in the game, and make money when we weren't screwing around. Never griefing, only earning. Easily my second favorite online experience ever.
After that, NIER, Metal Gear Rising, Marvel 3 all are strong in my memories as well