I was an MT in Vanilla and pretty much all the fights in MC were tank and spank and boring. BWL started to change the formula for a Tank. This frames my top 3:
3. Vanilla Onyxia: Threat mattered, especially Phase II-III transition. Being able to make that transition smoothly was an accomplishment and killing this boss was also very hard at the time.
2. Razorgore: This was before raid strats/videos were readily available and my job was to break the eggs first and then kites the adds around the room. Swiftness Potion, Nifty Stopwatch (still in my inventory because of this), and Piercing Howl were abused for this strat to work. Took us weeks to get the first kill and it was epic when it happened.
3. Ossarian: No one seems to ever remember or mention this boss (or raid for that matter), but it is my favorite boss of all time. This was so freaking fun as a tank because it was so different. There was so much movement and it took skill to make sure you found the next crystal to break in time while avoiding tornadoes. I had to pay attention the entire time to keep threat while moving in the proper direction. Very challenging and fun. You also had to have an OT who knew what they were doing as they had the same responsibilities as the MT, which was rare at that time in the game. Healers had to be on point too. It was really what co-op gaming is all about.