I think TBC simply because of the community back then. I mean my friends list was near-full with random people I'd met doing HCs with or got talking to after a Halaa battle. These days I rarely even speak to anyone outside my guild. There is the odd occasion, but it's all about who can be the biggest nob in LFR or who can AFK the most without being kicked from the random HC.
Being in a group used to mean something, it got worse past TBC and it was a great factor in fun for me. Not to mention things like Hand of A'dal, whenever I see someone with the title I think oh cool he was there when people cared. When I see almost any other title... I just don't care (maybe shit like Firelord pre-nerf etc).