Wow this is a really tough question. Recently I realized that out of all of the H!P Groups and solo artists Gocchin has the most albums and singles that I listen to everyday. She has some great dance stuff, hard stuff and ballads.
I guess if I had to pick one song it would be Sayonara 'Tomodachi ni wa Naritakunai no'
I think that all of the tracks on Goto Maki Premium Best 1 truly are some of her best music and I always have this album loaded on my mp3 player.