While I don't know an immense amount on the subject, here are my thoughts. :>
I first found Vocaloid back, I believe, on March 17th of last year. It’s a pretty big part of my life now, although initially I didn’t care for it all that much. After giving it a bit of time I started to really like some of the stuff, though, but I didn’t really find a Vocaloid that I could call my favorite and that I consistently enjoyed until around late August of last year when I discovered Megoid/Gumi. That was when I really started to like it.
My top favorite Vocaloids currently would be Gumi, IA, Yuzuki Yukari, and VY2. My favorite producers as of now are AVTechNO, Deco*27, Hachi, wowaka, and Kagome-P. One thing that I really like about Vocaloid stems from the fact that all kinds of people use the software – it means that you get such variety in style and experience level. This means that it can appeal to more people, I suppose. I know I wasn’t initially a fan of much of the super popular stuff, as I wasn’t used to hearing that genre of music much in general, so I looked for things more tailored to my tastes. It’s essentially just another instrument, and so it’s also important to keep in mind that some people might not be as experienced with that instrument as others, and some people will use it differently. I like that about it. Being used by a myriad of people, it can cater to a myriad of tastes, and it also means that once bad experience doesn’t reflect necessarily on the whole, as all the people who use it will be using it differently. ♥
I’m one of those easy to please types that listens to virtually anything if it sounds decent to me, so I like a good mix of Vocaloid songs. A few of my favorite or most-listened-to songs would include (keep in mind that these are all just mixed styles and the like, some are heavier songs for heavier days, others are just kind of cute) Wrist Cutter, Mozaik Role, Coward Montblanc, Panda Hero, Killer Lady, Love Droid, 5150, Soul Breeze, Beyond the World of Glasses, and Solitary Play, to name a couple featuring Gumi; Twitter, Desire, Tear, Alice in Black Market, innocent girl, SOU, ZOU, NEL, Two Breaths Walking, Sleeping Beauty, Lost Story, March Rain, and Arpeggio to name some featuring Hatsune Miku; Leia, Corruption Garden, Interviewer, Dye, and Fuyu no Sakura for Megurine Luka songs; Remote Control featuring Kagamine Rin and Len, Tiny Paradise featuring Yuzuki Yukari; and also an excellent cover of Katayoku no Tori using Mew rank into my favorites. I guess also Matryoshka, Intently, Headphone Actor, and the/day/after should be mentioned among my favorites. Certainly it isn’t a fraction of what is out there, nor a fraction of what I listen to.
So there you have it – all kinds of songs for all kinds of occasions. That’s the main reason I enjoy it so much. My shallow side enjoys it some for the adorable appeal of the avatars, too…
but another, truer reason that I like it is due to having a good respect for the people that make the music and use the software, making these products come alive for the rest of us.
:>
DDD" It makes me gag x.x Anyways, I like listening to the less famous ones, like Gumi, Neru, Lily, and Teto. 
