We've heard of World is Mine before. As well as some songs from the Evillious Chronicles series (ex. Daughter of Evil, Servant of Evil). Also, Rolling Girl, I=Fantasy, trick and treat, Crystalline (the song by CircusP and Crusher-P), Just Be Friends...we've heard of them all more than once.
Since the songs mentioned above are famous ones, some included in the Project DIVA games, some that have entered the Hall of Fame, some that has earned over a million views, why don't we talk about Vocaloid songs that are really underrated? The ones that people have never mentioned before or hardly talked about. The ones that when you ask people if they know those songs, they will reply "No". The ones that have hardly even reached a hundred or a thousand views.
Feel free to list down songs that you believe are so underrated that they deserve to gain more attention. We can also talk about those songs as well. I'll post mine later, though be prepared. It's gonna be quite a long list.