there are bad synths and plugins??
imo even the cheezy scope synths(a real cool class of sounds regardless of the platform) have a little more zing to them than their vst counterparts. it really depends on what you want to do though. a
musicain will take a sound, any sound and say, "cool", and make music. sometimes though, you want a
particular sound. then there might be a "best" choice.
as i say, the "best" sound is usually the original source. well, not always, but usually, so if you are asking about the minimoog, for example, then yes, scope sounds better than vstis(more like a real, "fat" hardware mini). generally, a scope synth will sit by it's self in a mix better compared to it's vst counterpart. for unique vst synths, the vst being the original source, well, vst would be the best(and most likely, only) version.
if you are
really into synthesis, the scope modular with flexor has no vsti counterpart. a computer's cpu would not be able to even
think about running something like that
and a sequencer full of tracks and plugins!
scope is more like buying a bunch of hardware analog synths and a modular(that would take up half a room if it wasn't on the card). scope is virtually the original because dsp chips are what run most modern synths! it's not really comparable to most vstis. most vstis that are comparable in sound quality to the more powerful scope synths(is there a vsti equivalent to solaris or prowave?) are the more expensive emulations and also are very heavy resource eaters, so in this way, scope is actually a very good buy, a bargain. it's like buying actual gear. it will extend the capabilities of your computer. it's a dynamite soundcard as a bonus.
once again though, it depends on what you want....