Some thoughts on the last couple of years.
Ulduar without question. The instance is stunning to look at and gets off to an atmospheric and impressive start with the vehicle fight. There is nothing else in the game to match the look of Kologarn, XT002 or Mimiron. I also like the genuinely new types of encounter - particularly Mimiron and Yogg Saron. These are really interesting and well put together.
Most difficult encounter
The encounter I've wiped most on without clearing was Anub'arak hard mode in ToC10. We stopped trying it when ICC came out, so while we could have gone back and stomped all over it with our new gear, we haven't. It's not really the same without the challenge I think.
We tried the fight for about four raids without success, and came close a few times. We would regularly come unstuck in the kiting phase with stupid deaths - people would use aggro-dropping abilities without warning the raid, so the boss chooses a new target without resetting speed, or people would try absurdly over-ambitious kiting strategies, or people would die to the adds. It was so frustrating.
In phase three I don't think we ever really nailed the healing - we had too many people being healed too high so that boss healing was massive.
In context though, Anub was nowhere near as difficult as Arthas. Arthas was a worthy end boss for the expansion and probably the toughest fight I've ever seen in any expansion.
Least fun encounter
Ugh, Malygos. Having to move the boss constantly away from the sparks, while allowing the melee to stand in the power circles, and keeping aggro ahead of massively buffed dpsers. An absolute nightmare. Once you'd done all that, you were at the mercy of people not dying on the drake phase, which they invariably did.
With hindsight, part of the problem could have been that prot warrior threat generation was dreadful in 3.0 warcraft. If we'd had the dps buffs we have now, keeping aggro could have been much easier.
Most fun encounter
Faction champions I think. My job was always to bully the healers with stuns, interrupts and fears. It was an excellent fight and you really felt that you were contributing if you could tie up one of the healers more or less constantly (mwahaha I own you Mr Priest).
I like fights where spell reflect is important too.
Most rewarding boss to tank
XT002 hard mode / Anub'rekan. We got this achievement around the same time as ToC came out, so we were all still in ilevel 219 and 226 gear. We solo tanked it, meaning that I needed to keep aggro on the boss, and also grab the sparks when they spawned. We also (and this is perhaps my favourite part about the fight) had a *clever strategy* for the fight, which involved special positioning for people who got lightbombed. It was my first hardmode fight, and was the time when my tanking stepped up a bit to take on the MT role in progression content.
Anub'rekan is in there because it represents one of my fondest memories of the expansion. It was my first WotLK raid boss, and with my 21k unbuffed health, I just could not survive leeching swarm. However fast I ran I would always get a few ticks and the poison would kill me before the kite was over. So I made a plan - I got one of my healers to stand 25 yards away, and I macroed intervene onto her to a key. I then immediately intervened onto her to escape the swarm and avoid getting any leeching swarm ticks. I love things like that - and that is what ties XT002 hard mode and Anub'rekan together - both fights we succeeded by coming up with a special strategy to beat a particular mechanic. This is perhaps one of my favourite aspects of raiding. It must make it frustrating to be in one of my raids, since I'm always wanting to try some new tactic or strategy, or trying a new positioning idea.
Maybe raidleading in 10s is like playing an RTS game on a grander scale. We have units (raid members) and between us we come up with a plan of attack for them. My RTS units rarely go afk for a smoke though...
The LK kill. After about 85 wipes and 3 months it was a little emotional.