Pre-TBC
Best instance: Princess run in Mara ~ this is cool, and some nice memories attached to it. For my first char Princess was the equivalent of a raid boss!
Worst instance: WC ~ so big, so boring, crap loot and laggy.
Best raid: UBRS ~ don't forget this old gem guys! It was awesomely fun and intense, so many memories and always a great bunch of guys and some hillarious wipes to boot. The Beast's epic hide amazingly dropped like 90% of the runs I went, but despite often being the skinner, I never got my hands on it
post-TBC
Best instance: The CoT instances have the best vibe, but SL Heroic is the most fun and challenging. Ramparts is also hella cool for a lowbie group.
Worst instance: Most of the TK instances.. can you say yawn, and the look doesn't fit WoW.
Best raid: Karazhan! Call me noob but the atmosphere is wonderful, I love the pseudo-medival story about Medivh's tower and all the humour, and each boss is so unique and original.
Worst raid: Magtheridon or Gruul right after he Ground Stomps on a bad night.. stresses the hell out of me.