As a rogue I always loved the Vaelastrasz fight in Blackwing Lair, immensely fun just doing insane amounts of damage and (almost) not have to worry about threat. I also love most bosses in Karazhan, the Chess Event in there is especially different.
go BWL nor in HELL FIRE peninsula.. good drops there.. violet cool! more armor +resistance to shadow, fire
honestly i love almost every fight in wow, ragnaros was my first good one, then everything changed when i got to razorgore/vael(different strategy then just tank and spank) and nefarian was crazy, but my favorite fight had to be in naxx, the guy with 4 guards where you mind control the guards to tank the real boss edit: razuvious was the name
if you want good drops. try illidan ..he drops blade of azzinoth. and a great staff.. and guys try also archimond. aheheh
I hated Molten Core; you had endless amounts of trash which made the instance take five times longer then it should have been. Ragnaros was one of the worst designed encounters when you first got to him because it was near impossible unless everyone had max fire resist gear, but they eventually nerfed him so the splash damage could be resistable. Umm?? You're crazy. Talk about one of the worst designed encounters because it was ALL about threat. If a good player went all out without using any -threat abilities, they would have pulled aggro. Blackwing Lair (BWL) wasn't as bad after they fixed everything, but going in there when it was first released was one of the worst tested instances they've ever released. The first boss having the zone lag out and causing the server to lag, Vael causing the server to continuously lag and only having an hour to attempt him before he disappeared for the day, sometimes the vael gate not opening, etc. After they fixed a lot of the bugs, if they reduced the amount of trash from after Vael (egg room anyone?) to Nef, it would have been a better instance. Zul'Gurub was one of my favorite instances that we were able to test on the test server for some of the developers, this is when they introduced the start of miserable rep grinds but they also introduced the amazing class/quest objects for class specificed armor; this was a good thing to have when you were used to getting the same class items in raid instances every week. The outdoor dragons were sweet because one of the things I missed about older MMOs were the outdoor raid bosses where people could wipe each other and fight for the items, instead of everyone being able to obtain things weekly. The funniest thing I ever saw was when we used the shadow reflectors on one of the green dragons that has an hp % debuff and it killed the dragon. This made pvp servers a tad bit exciting, considering the huge lack of pvp orientated content in WoW at the time. The AQ gate quest (the chain to open the gate, not the farming mats part) was probably my favorite time in World of Warcraft, ever. Doing a chain of quests that no one else could do and trying to find hidden things in the world that no one had done before. Think about having a quest chain that involves hunting things that no one has ever done before and the game becomes a lot more immersive, a thing that MMOs lack nowadays. AQ 40 was similar to Molten Core, having stupid amounts of trash and the bosses were relatively easy (my guild went in when it was first released and we were able to two shot most of the bosses, while one shotting a couple of them). We were also one of the first guilds to kill the first version of Viscidus (everyone thought he was impossible without max resist gear, but all it took were engineer explosives and healing throughout maybe one or two stages of freezing him) Naxxramas was probably my favorite instance because of how they separated the wings, giving you a choice based on how you wanted to learn the content and how you could take a break from a boss to go try to learn another one; although I think this was poorly done because a majority of the player base never got to try it. Some of the encounters inside the instance were the best designed pre-expansion because they involved good strategy and execution from every individual. Although there are a lot of idiots that play, good execution should be required and bad players just have to walk away, hard bosses should be hard. I pretty much enjoyed all of the expansion content except for some specific things that stick out in my mind right now like the trash in tempest keep, but my favorite encounter of all time is definitely Kael'thas Sunstrider - it's a long fight with some amazingly fun concepts and lots of graphical involvements (like the room shattering and you able to see the outside / nether)