It is really easy to get a Prophet 5 to sound good. I've found that the challenge is actually to reign it in for multi-part compositions.
I think this is my main point too. Getting a synth to “sound good” (whatever that means) on its own is something very different to a synth part working beautifully in a track.
In regards what a “sweet spot” actually means, I’m not sure I get that either. Is it simply just a “good sound” from a synth? Or is it something to do with how you balance the filter and resonance for example? Never really made sense to me! I guess it’s so subjective.. there are so many sounds a synth can make, and some are right for some contexts and not others, depending on what you are doing
There are probably mutliple rabbit holes I could go down with this topic that would rope in concepts such as variance depending on what genre of track I feel like creating on a given day, etc. but I will try to keep my ideas reasonably succinct....
... my workflow usually begins with one synth that acts as the "idea springboard" for everything else. It's usually not a bass synth (although sometimes can be)... it's usually a poly synth where I can experiment with chords, though it may end up getting played monophonically.
That "first-in" or "idea" synth is critically important from an inspiration standpoint because it tends to define the direction for how the rest of the track develops. Ironically, that synth may not even be used in the same way later in the track (the MIDI I created with the "idea" synth might end up being sent to and tracked from a different instrument).
For that initial process, having a synth with tons of sweetspots is crucial, because as I'm searching for where to go next, I might rapidly tweak knobs in order to achieve a vague goal I have in my mind, and I don't want to spend lots of time getting lost in minutia an obsessing over perfecting the qualities a single sound.
During that process, if I am working with the type of synth where new ideas just fall out of it and auto-magically sound good, it has a significant impact on inspiration, so a "plentiful sweet spot" synth is ideal for this role; this is a very different benefit that I get from the type of synth that is deep / complex, but rewards the time investment spent on it ... that's fun too, I love sound design, but it's a different role for an instrument and a different workflow with regard to track creation.
So to answer your question, in my case yes, a sweet spot on a synth usually means a great sound coming from that synth when I am
soloing that instrument -- not same as how it sounds in a mix or plays with other instruments. Sometimes it's about filter/resonance, but for me it's more about how quickly that synth can be tweaked to go from a broad/vague idea of a sound category to a more refined/focused version of that sound, and still sound good (meaning inspiring, not harsh, whatever) across a wide range of parameter changes as I refine it and narrow down the idea.
Some synths are more tempermental and feel all over the place when tweaked; they have their place but don't typically get a primary role in my workflow.