I was in a raiding guild that did ulduar, i never got to set foot in the raid when it was current...
That being said, when I DID enter the raid, it was a damn masterpiece. Not only was is a new leap in titan lore, it was a beautiful prison for an old god, and showed us the capabilities of the Titans and their view of Azeroth when it was reshaped.
It tied all of the keepers lore from all of Northrend together into a fantastic culmination of a raid. And fully explained who Yogg-Saron was and how he was able to achieve what he did.
And aside from the lore perspective, the raid encounters were phenomenal, every fight was challenging yet simple. The bosses themselves had unique character, and each downed boss felt like it would have been an achievement in progression.
And last but not least, the hard modes completely revamped the encounter every time you attempted them. The bosses literally had dps checks built into them to prevent people from getting cocky with their raids. Not to mention key changes in the boss encounter that pretty much changed how the entire fight worked entirely.
And 0 lights in the darkness is still tough as shit to master. It can easily be out geared now, but back when it was endgame it would need a completely competent team just to get the first two phases started without someone going insane.
ToC, and everything afterwards pales in comparison to how well and polished that raid felt. everything had its place and nothing was broken or missing that could improve it to its highest potential.