Hardest. Poll. Ever.
I have a deep love for the Deadmines. VC brings back such delicious memories. I love the pirate motif. I first ran with friends and that makes an instance that much more fun. It was also my first and is a short, fun instance filled with notable quotes. "YE LAND LUBBERS ARE TOUGHER THAN I THOUGHT!" It never fails to make me laugh as we goofily imitate all of the characters word for word. And really, who can forget about Cookie?
Wailing Caverns was amazingly beautiful. I spent my time leveling fishing here from 125 skill to 300. Two words: Deviate Delights.
Shadowfang Keep has the BEST music. Absolutely amazing music. I loved going here for my paladin even though it was quite the trip for her level.
Finally, Scarlet Monastary was so much fun. The first instance that featured multiple instances within one heading, had a great storyline that continued well into warcraft lore, had some amazing pieces, and could be run again and again without seeming to bore a group. This is also an instance with some quotables ("Release the hounds!") and hits about that time where your class just begins to feel like your own due to the customization of the talent trees. You have 31 point talents by this time, or most likely can access them, and each player begins to fill a role as opposed to merely zerging through the content.
I will have to vote for SM, because that's when I decided healing was absolutely it for me.
Being sad is the same thing as giving up.