There's two answers to that.
- - How easy they are to use for your purposes. Buy an easy-to-use Vocaloid. For example, Hatsune Miku is traditionally the beginnier vocal in Vocaloid2 for Japanese as is Sweet Ann in English Vocaloid2. You should try for a Vocaloid 3 at this stage more since the software is in developement and Vocaloid2 is not. Of this, Aoki Lapis, SeeU (though only for her Korean and due to lack of choice), Oliver (again due to current lack of choice) and Bruno are usually taken to be amongst the easiest ones for each language, .
- - Buy whoever you want. Buy a Voicebank you'd enjoy using.
The first implies if you are trying to take this seriously, make something of the product, the second implies only if you do it for fun. Either way you should read up about vocaloids and consider things and issues such as language.
If you are after a English capablities, then you don't want an Japanese vocaloid, you need an english unless your after the accented sound. Not sure? Go for one that does more then one language like Luka.
Also,look out for upcoming releases, sometimes holding back for the tright vocal pays off.
Also, don't presume that you can buy a Vocaloid and use it for HQ results if you don't know what your doing with it. For example, we encounter fans who have presumed their Japanese cover song of "Magnet" with a Vocaloid such as KagamineLen or Lily or any other Japanese Vocaloid is good, when in reality a native Japanese speaker can tell them more in-depth how awful the cover is.