[Update #2]: World of Warcraft: Cataclysm will be getting the Classic treatment and many new improvements in 2024.
[Update]: Good news, everyone! Icecrown Citadel is now live!
While there are some that will give this title to Ulduar, in my humble opinion the most legendary raid in all of World of Warcraft Classic is Icecrown Citadel. Besides having a bunch of fun boss fights to smash your face against, it also represents the climactic end of the long-running Warcraft 3 storyline, as well as Arthas as a character. Well... if we ignore his brief appearance in Shadowlands anyway, which in all honestly we really should.
With all of that in mind, I'm very pleased to say that Patch 3.4.3 will be heading over to Wrath of the Lich King Classic on October 10th! Patch 3.4.3 will bring with it three 'new' dungeons, the Random Dungeon Finder, an improved quest log, a new tier of challenging Titan Rune dungeons, a new Collections UI, and the list goes on for a while.
Naturally, the definite highlight of the patch is going to be the opening of the Icecrown Citadel raid itself, though that won't be happening until October 12th. The reason, as you might imagine, is to give everyone a little bit of time to explore all of the new content before burying themselves in the raid and the hunt for the Shadowmourne legendary axe.
While the exact patch notes have not been released just yet, you can get a pretty good idea of what's coming and what's being changed through the PTR notes over at the WoW Classic forums.
Enjoy, and I'll make sure to let you know once Patch 3.4.3 goes live on October 10th.