The hard part about answering this question is both a positive and a negative and its the same thing, the variety is mind boggling...the downside is the cost of trying them all
I think there are stinkers and gems across stock and modded whatever the brand...but tone is in the ear of the beholder - I have hated some modules people applauded.
Here is briefly my opinion on Synergy so far:
SLO - #1 reason to own SYNERGY in my opinion, 100% still right out of the box, the SALVADO can be dialed in almost the same but is more aggressive/not quite as thick in the mids. The Salvado has two channels and some switches...can do CLEAN/edge of breakup on ChA so edges on flexibility.
TDLX - I have been happy with TDLX in whatever format for CLEAN (Randall, Egnater, modded.) I'm not a pedal gain guy but none of the Randall stuff seemed to be that great a pedal platform and the gainer side a bit raspy, have not tried the SYNERGY as a pedal platform. Hands down favorite modded clean module - Salvation LONECLEAN.
SAVAGE - I had it one day and played 10 minutes, it's a keeper...not sure I remember the ENGL amp being this noisy and I would always turn off the noise gate there, my Powerball had tons of hiss and this module has gobs of gain....12 o'clock is a *lot* in Hi gain mode. I believe Blue/Red are voiced differently and you have Hi/Lo gain per channel. Honeymoon period but this is great. (Reference points ENGL PB/BM/SE and SALVO ANGEL)
PLEXI - I don't even remember the name of the Randall module but have had variations of Plexi and SL from all the modders...by far the best sounding was a Friedman Brahma (more later) and I did not expect to like this module at all, being a gain whore. Imagine my surprise when I found I could coax out some great tones and really puts this into quite saturated gain...it's pretty flexible with I/II gain controls per channel..doesn't really do clean so imagine it's higher gain than the Marshall.
VH4 - Love/Hate relationship with this module, every modded take I had on this has been heavy as **** but noisy. The Synergy is way more refined but just when I think I have a tone dialed in..I come back to it and hate it...my brain has something against the way Diesel is voiced I think, so that's my problem. I wish the SAVAGE had the Dual EQ of this module, just for flexibility, but I believe Blue/Red are voiced differently and you have Hi/Lo gain per channel.
METROPLEX - I listened back to back with the Metropoulos amp and this one nails it, just haven't found something I'm comfortable with yet. It's up for sale or trade
OS - had 10 minutes with this and boy is this a fun module, no idea what I'm doing with OD RATIO and LEVEL but this can be either touch sensitive or bleed into lovely harmonic feedback on a whim. This module has mids for days....perfect noodle guy. Honeymoon
With mentioning that I had Salvation/JF versions of this....they don't compare, this one seems to be built very differently and I think building OS/OD onto the original module platform had some compromises.
BE - Everything Friedman touches is gold in my opinion and the BE/HBE I pre-ordered along with the SLO. I have no reference to his actual amps. I had a Friedman modded Brahma that I should never have sold..with the NOS long plate Yugo/Phillips 7025 it had this snarl and growl that was everything. The SYN BE sounds relatively polite or refined by comparison.
HBE - I had been on the verge of selling this, had several Friedman modded version built on either Randall or Egnater platform and they were balls out aggressive, the Dual C45/HBE mod built on the Egnater platform was killer. This synergy one is again refined...but have been slowly dialing it in and I think its staying as counterpart to the BE
AC - no experience yet and I inly mention it because I have had a love/late relationship with VOX in the modular system and I had EGNATER VX, JF, SALVO versions...never bonded with any of them. I recently repurchased a DUAL EGNATER VX and found I could dial it in very nicely. Is it VOX exact? NO....but is well in that ballpark but with more gain available so am happy.
Honorable mention here to the Lynch Sigs....I know Friedman had a hand in some of these and they are all great tone out of the box...though puzzlingly Dave has offered mods to 'correct; Randall production errors and the only way to find out of it's correct is to send it to him. The Friedman modded version I've had of Scary and Brahma were golden...so I guess there's something to it.
SuperV/Scary/Grail/Brahma/GTO all offer something good IMO.
Some have commented that SYNERGY (and Egnater) shared EQ modules don't really offer much, well they offer you the ability to have a low/hi gain and different volume onboard but still retain the same core tone...somethins that's ideal. Maybe a Dual EQ offers more flexibility and some modules offer a slightly different voicing on Blue compared to Red channel....
The modded module world offers something different and in some cases greater flexibility with switches but at a cost (donor+mod fee) and I have many a modded module I love.....they are different routes to a similar solution.
I say just seek the tone you love, you might find it in unexpected places....stock or modded, whatever the brand.
I think Synergy is still finding its feet, look how long it took Randall to bust out the Lynch and KH sigs, but I think Synergy is really coming into its own with the modules just released and on the horizon.
It's frustrating to wait but am excited for the coming BOGNER stuff for sure...XTC was another money pit I spent on Modded modules, I'm betting SYNERGY nails it out of the box.
Oh I forgot the Fryette too...though I already have killer VHT tone in a Dual Modded Module format.
NOTE: One *mind boggling* thing about the SYNERGY stuff, why on earth did they take away the external power tube biasing points??? Since they dropped the price of the SYN-50 to almost reachable, it's pretty much the only thing keeping me away.