One of the reasons for this is cillia, a 16 year old girl from California according to her Nico Account. (YouTube) She has become known for her use of Ritsu's newer voice bank, but she had also covered songs using Teto Kasane, Yuki Kaai, Lily, Yufu Sekka, and Len Kagamine. Her tuning of the voice banks is powerful, and she's releasing new songs often. She both creates UST and edits already existing ones. I can only be amazed at how nice UTAU sound now and how far they have come. There are many songs that I can't stop looping, and you won't here me complain about the increase in amazing UTAU songs from cillia and others.
At the time of writing this, the original "-ERROR" has been seen over 154,000 times on NND. cillia's cover with Ritsu has been seen 260,000 times, more than the original. The original "+REVERSE" has been seen over 83,000 times. The cover, over 116,000 times.
Enough introduction. On to the music!
The song that started it all and took everyone by surprise. If you check the info, cillia posted a link to download a "corrected" version. (I have trouble telling the difference between the two.)
A few days later, cillia did it again with this song. This is one I can't stop listening to.
more of cillia's works
I was a fan of this niki Lily song, and I was ecstatic when cillia covered this one as well. I love the "AAAAAAAAAAH" parts.
【歌愛ユキ】 ぼくらのレットイットビー 【カバー】
A little break from Ritsu with this Yuki cover. Yuki sounds really gentle in this.
I think Ritsu sounds a little more robotic in this one possibly from the effects given to the voice, but it's still really fun.
"A Tale of Six Trillion Years and One Night"
This kemu song should be familiar as it has been on the rankings for a long time. Both Ritsu voice banks are used. The older voice bank is the very soft parts, which are a treat to hear.
"A Tale of Six Trillion Years and One Night – Piano Ver."
And this is the same song only done with piano and Yufu Sekka.
"Mysterious sexoB elttiL
Tetoris sings this one and made me a fan of the song. Ted is used for harmony.
Sora wa Takaku Kaze wa Utau
Now for a flip. cillia makes an UST for her friend to use for Lily. (Note: This song and the next are not on her YouTube account)
Here is another anime song, originally created by supercell. (Who here remembers when supercell used Miku?) I like Ritsu's English, and compared to the previous songs, Ritsu has a gentler voice.