I'd say it has something to do with the featureset, combined with the accuracy of the filters (that is, both tracking accuracy and whether the typical side effects of filters impose something to the signal).
I tend to view vocoder plugins like filters - the more, the merrier. The differences, in features and how many bands and so on mean a greater variety.
There really isn't a vocoder plugin in existence - at least not yet! - that I'd consider being the best. If there was, it'd be a cross between the features Logic's Evoc and NI's Vokator, intelligibility of Starplugs Vocoder, usability of several of them, referring to having a good enough carrier generator on board... and it'd go further, allowing some further effects selectable and individually controllable with each band, it'd do tricks the old analog ones allow, such as a mod matrix style features for routing different carrier bands into different modulator bands and so on... and it'd let the user control the filter parameters on each band as well. It'd be a bigass CPU hoggin' pig of a plugin, but worth it
So, who's up for the challenge?
Regards,
JMH
