That's a hard one too, there are many options here. I love to waltz, and I took some ballroom and some swing lessons at different points in my life.
Of course, I also just enjoy shaking my bum on anything with a good rhythm (be it dance, pop or hip hop).
So I'll pick a song for which I know the official choreography, to which I've danced many many times when it first came out (especially at the aforementioned Fêtes de la Madeleine) and to my surprise as recently as last January at a Beach Party in Punta Cana: La Macarena by Los Del Rio!!
It's not a music masterpiece, I think we can all agree, but it's a lot of fun to dance to ;)
PS: I skipped yesterday, so I'll be back later today for the next category