It depends on several factors that should be took into consideration.
First of all, one must decide which version of the VOCALOID editor it is that one wants. The latest version is VOCALOID4 which is compatible with voicebanks created for VOCALOID3 and VOCALOID4, though the former will not have access to the GWL parameter. It used to be possible that VOCALOID2 voicebanks could be imported into VOCALOID4 but support has ended for it. VOCALOID voicebanks are not at all usable in the VOCALOID4 editor.
Next, one should regard the language that the VOCALOID has been designed to sing in. Bilingual VOCALOIDs like Megurine Luka and YOHIOloid are good choices for experimenting with one's own language as well as a foreign language. However, do note that the VOCALOID's primary language vocal (Megurine Luka: Japanese; YOHIOloid: English) is significantly better than that of their secondary language.
Note: one should not presume that one will get perfect renditions of a language the VOCALOID's voicebank is not made for. For example, Japanese VOCALOIDs are not capable of singing as well as an English VOCALOID in the latter's language. This is merely a myth, and can take hours to force the VOCALOID to pronounce anything that sounds remotely like the said language.
If one is to purchase a VOCALOID based on their language, there is a way to eliminate the number of VOCALOIDs to choose from. Typically, the higher quality VOCALOIDS are easier to work with due to having not as much errors or glitches. The following are VOCALOIDs that are renowned for being fairly easy to use and are on newer versions of the VOCALOID engine.
- Japanese: VY1v4, Yuzuki Yukari V4, Nekomura Iroha V4, Megpoid V4, Rana V4 (has been specifically made for beginners), Kagamine Rin/Len V4X, Fukase
- English: AVANNA, CYBER DIVA, DEX
- Spanish: Bruno, MAIKA
- Korean: not applicable
- Chinese: Yuezheng Ling, Stardust
Before one rushes out to purchase a VOCALOID product, there are a few additional notes that are worth considering:
- Consider the style of its vocal and the professionalism. Some VOCALOIDs come with notes about optimum vocal range, tempo, and the genres they are best suited for. There are specialized vocals such as Sachiko, who has the best performance in enka, R&B, soul, blues, and ballad, and flower, who is best in rock and metal.
- VOCALOIDs whose voices are either voice acted or not sampled from a professional singer's, like Hatsune Miku, MAYU, flower, or RUBY, cannot produce produce professional singing results. Generally, vocals that have not been voice acted, like VY1, Yuzuki Yukari, DEX, or DAINA, sound more realistic. Other vocals that are supposed to be faithful recreations of singers, like Camui Gackpo, IA, Sachiko, or Fukase, are known for both professional and realistic singing results.
- With each VOCALOID having certain strengths and weaknesses, it's not always easy to pinpoint which VOCALOID to go for. Therefore, the best method is to sit down and listen to many songs sung by the VOCALOID and determine their vocal capabilities.
- Do not assume that it will be easy to make a vocal sing in a certain style, because producers often edit vocals to the point where they will fit that song. Hatsune Miku for example, has been made to sing every song genre.
- Keep an eye out on upcoming vocals that are to be produced by any of the companies, the vocal you are after may be released.
It may turn out that one is better off not buying a VOCALOID at all, but that's another issue one will have to decide for oneself.
Finally, if one intends to get into VOCALOID more seriously, including buying more than one vocal designed for VOCALOID, then one must plan one's purchases, as VOCALOIDs are not cheap.
As for where one would go about buying VOCALOIDs from, many are bought from their own studio pages such as Zero-G or PowerFX, some are able to be purchased from eBay or Amazon. They can also be bought from the official VOCALOID shop.
Alternatively, one can ask at a forum for both advice on new vocals or for help getting them. All opinions are subject to change depending on individual, version or voicebank.