I was a hardcore raider through all of TBC and into the beginnings of wrath. I didn't even finish every hardmode in Ulduar but I knew it was the best raid dungeon Blizzard had ever designed until that point. People love to talk about TBC raids being great, but honestly they were some of the worst designed raid instances. Karazhan and Sunwell were really the only two well design raids. Everything else was shit. Ulduar was well laid out, evenly paced, well balanced, and had content for every spectrum of player. The noobs could farm the easy mode bosses, and the hardest of the hardcore had 0 Light Yogg to tackle. The hardmodes were interesting and introduced new mechanics. The instance was NOT LINEAR, and this was huge. They took a page out of Naxxramas design for this, and honestly I wish every raid was this way. The hardmodes were no joke, it was a significant event when the first guild downed Firefighter, 0 Light, Algalon, tc. It is still to this day the most varied and interesting raid when it comes to boss design, instance design, and overall accessibility for everyone.