If 2008 then I would probably pick that.
Still 2009 is a close 2nd. Wrath of The Lich King had been out and I developed into a raid leader of one of our 10 man groups and partial leader in our 25 man group. Our guild was peaking at around top 10 for Horde on our server which was a substantial achievement for our casual raiding group. I was doing well in pve and pvp, and other guilds even offered me positions, but I passed. Dungeon Finder unfortunately released in December, but it was only one month in 2009, and back then I enjoyed it. I also enjoyed Console gaming despite being hardcore PC, with Uncharted 2, CoD, Dragon Age, Demon's Souls, and many more.
2014 and 2018 are next, with the others lagging behind.