If you would like to try one of the V6 banks, I have had quite good experience with
Nurse Robot Type-T! I've also heard the new
IA :[R] 
is good, and she kept the tone of the older V3 bank too. These voicebanks can do multi-language singing, yet it is trickier to find where to tune them in the editor.
If you have any interest in experimenting with phonemes / phonetics I can't recommend
MAIKA 
any more, and she has a unique tone and doesn't need much tuning to sound great!
For English banks (that aren't abandonware :( ) I would recommend
Gumi English 
and
CYBER DIVA 
. If you can't get the V6 editor, then
Macne Nana 
. Macne Nana (as much as I love her) has a very unique tone and has been known for being sort of finicky. I can't find her on the VOCALOID site (and the others are blocked at school, so I can't verify if she is discontinued.) but AVANNA

is also very good, with a unique tone of voice!
For Japanese I would recommend Rana (Same situation as AVANNA)

as she is designed for beginners and has a cute voice! If you would like someone more versatile I would recommend
Otomachi Una 
as she has a complete set with two different voice tones. If you'd like a cuter voice than that then I would recommend
Aoki Lapis
, and if you would like a softer voice I would recommend
MAYU
.
For Chinese (I'm just going through all the languages) I would say either
Luo Tianyi 
or
Xin hua 
. I don't have any experience with these personally, but I assume Tianyi is easier since I have seen her more in the mainstream. She is basically the Chinese Miku from what I've heard.
Finally, for Korean your option is
UNI
. She has a unique pop tone to her as well!