My happy dances are all about reconnecting with long lost relatives. I can think of 2 major happy dance events. The most recent was connecting to cousins in Lithuania. I've written about that here.
My best previous happy dance was about connecting with relatives on my dad's side, through the Chicago Genealogical Society's "Chicago Genealogy Finder." A long lost cousin wrote a letter to me after seeing me listed with our common ancestor, Timothy Whelan. Unfortunately, the address listed was over 10 years old, and I had moved from San Diego back to the Chicago area by then. My distant cousin is a librarian, and a very resourceful person. When her letter was returned, she found my current address in more recent membership lists, and tried again. We connected, exchanged packets of information and had some nice get-togethers including her mom and my Aunt Rita. Both sides of the family knew my great aunt Frances Whelan (also known as Sister Delphine.) I also have an autograph book that belonged to my grandmother Jennie Whelan, which was signed by Jennie's half sister Mary, the great grandmother of my long lost cousin.