My favorite raid was Ulduar, I was finally coming into my own as a raider. I had never really played TBC and had certainly never raided until a couple months after Naxx(WOTLK) came out. After thoroughly reviewing all of the raids that I took seriously(Firelands,ICC,1st Tier MoP,Dragon Soul and Ulduar), I came to the conclusion that having integrated hardmodes that you activated through doing certain things in the game, was probably one of my favorite mechanics of all time. The first time we got firefighter in 25m was quite the day for me. Plus, the raid had a hidden boss(we all new about him but it was still cool to think about) and it had optional bosses.
After taking some time away from the game and coming back only recently, I really truly enjoyed Ulduar seeing it as it was my first progression raid ever where I was hitting it the day it opened with all BiS gear. The variety of game mechanics that Blizzard showcased in the instance was beyond me. So many new things; shattering constructs in the water(I played a boomkin so I got that duty), a boss who shot laser eye beams on the ground, over 9000 Cats that 1 shot tanks on most of our first couple of pulls, the heart of a machine that no matter how hard we DPS'ed it was never going to die in the first month and a half the instance was open, and so many more little things that made the fights so enjoyable.
After going back and playing some of the old instances, I think I probably missed out big time on Karahzan and Sunwell. Alas, I was fresh to the game at that point and didn't know what the words "end game content" meant.
Now a couple of weeks after the new raid has opened and I am severely undergeared after missing a couple of the middle tiers, I am at least thankful that I have the option of LFR to supplement my gear. It has been a couple rough LFR runs, but overall I hope that I can get back into the raiding scene so that I can enjoy another gift that blizzard has given us(or that we paid for).
So what is your favorite raid?