Vanilla, without a doubt. Story time!
Oh goodness, I remember with serious fondness being a new player in the Vanilla days. The amount of hilarious things that'd happen, the friendships you'd forge, it was all epic beyond belief. Not the mention the PvP was epic beyond words. I still remember the day as a level 20, with my other friend a level 20, both of us sneaking into Horde territory as young night elfs. We were so confused by the game still, so new. The times we attacked guards and became flagged and ran for our lives by shadowmelding whenever a horde member would run by, then we'd run up the side of the barrens mountain and camp there literally all night, planning, trying to figure out how to get back home (we kind of uh, destroyed our Heartstones. Vanilla was a crazy time. Don't ask).
It took us hours how to figure out to sneak back alive while dodging patroling 60's. I'm not even joking, the amount of fun I had that night was the most in my five years of WoW. Then it became a downward spiral once TBC hit, everything felt bland. Even PvP was bland by then. 35,000 player kills and it was just boring. All that people wanted was to rush in AV, do the boring grindfest Arenas, and disturbingly over-inflated markets caused nightmares. Even my friend became bored after all that time and soon tried out raiding, becoming less and less of a friend over the years. What started off as a friendship between two level 10's in the Nightelf starting area, and became best buds for a year until TBC hit, soon became an annoying "do I really even want to talk to this person anymore?" kind of feeling. And by the time WotLK launched, all my best friends of those years had left. Every single one. Amazing friends that I had for years.
With a broken game, destroyed community, and abandoned by all my friends of years... I left. And have never gone back.
In my eyes, WoW died the day TBC hit. Rest in peace, World of Warcraft vanilla. You were buggy. You were confusing. But you were the most damn fun I ever had in my video gaming life. Take care, ol' bud.