Hydrasynth nearly beat out Syntakt for me but sometimes tracking down sources of clipping within the Hydra’s internal synth modules can be frustrating. And I’m not a big fan of the Hydrasynth’s reverb, you don’t get that same instant gratification you get when you whack anything into a long reverb on Syntakt.
But it was close because I can create insanely complex stuff on the Hydra so quickly. It is almost Elektron-like in that respect.
I haven’t had that many synths thankfully. I keep things that are fun to me and do not hoard things that are not, so my answer is the Monomachine.
I will try to elaborate and order matters.
My most fun synths are:
Sequential OB-6, because it is very simple yet very deep; I get a feeling that many people underestimate the possibility of cross modulation section. The only synth that would make me stand up from my bed in middle of a night and say “I have to try this idea!”.
Modal 008. Another simple synth from the surface, but it has also great sonic landscapes. And believe it or not, but it has its own sound. I bought it to feel my desire for Jupiter 8, hoping it will be sonically close. In result I have something that is kind of like Jupiter 8, kind of like Jupiter 6, but also - neither. This one comparison would lead to big paragraph of explanation, but I don’t want to dilute topic. From many synths that I own, it is one of few (Sub 37 is another one, closing the list) that has proper implementation of analog behaviour of envelopes, which is important and enables some unusual playing otherwise impossible to make (I mean that when you modulate sustain, envelope will follow new setting with curve dictated by decay, so in a way it’s like lag processor). And that all is wrapped up by THE BEST modulation matrix I have seen in a synth.
Novation Summit. I not only like the whole package, but also sound of individual components. I could have some serious drawbacks to mention, but let’s not go into this (I will just mention menu system). Generally it’s mostly hands on and with its sound it forms coherent package that I like very much.
Almost all my Electron gear, but in different mindset. A4, Rytm, Takt trio. Except OT - this isn’t that much fun as the rest, but I keep it for its creative potential, when my energy provides.
What’s interesting, while I have hundreds of HP of modules, eurorack never managed to hold me for prolonged time. There is something soothing in thought that I will be able to save my exploration and come back to it later and try it with different mindset, in different context of building composition.
My best sounding pieces of gear:
), Tallin, Starlab, Magneto and Mimeophon - just to name few that came first to my mind. And when I think about that now, I might contemplate on that list in near future, expanding it slightly and then shrinking my overblown collector’s disease display to this more healthier size. 
Now coming to significantly more used instruments…
Korg Prologue 16. My God, this thing strikes perfect balance for each element in chain, from oscillators to effects, oozing with character.
Modal 002. I actually think that in each own way Modal 002 has the best oscillators. They are NCOs, but I spoke with Paula, who designed them, and she confirmed my suspicion about their design. They are driven by clock that is effectively very organic VCO, one of the simplest one can make. This is why they are so pleasing. They are miles above the real VCOs that are in Modal 008, which might be surprising to some. Oscillators alone could sell me this synth and I am not kidding, because I have both - 002 and 001 - just to have this flavour in other place. VCF and all modulation of that synth are also great features. After OB-6 this one is second most used instrument in my setup. Pure pleasure to listen to.
OB-6. Yeah, it doesn’t sound that good to me as my Prologue 16 or 002, but it suffices in that department to deserve spot on that list. It still sounds great, don’t get me wrong, but lacks some traits that above mentioned provide and that are important to me.
Thank you for your attention.
Cordially,
Norman
The Syntakt + TD-3 MO combo has been the one for me lately. I really get into a zone when those machines are on and active.
Great question, I was just thinking about this the other day.
The funnest synth I own is the Arturia MiniFreak. They have jam packed this thing with so many fun goodies: user programmable LFO waveshapes of variable step lengths, cycling envelope with bendable shapes, creative and performable arpeggiator modes that you can copy to a sequence and save, performance oriented, p-lockable sequencer, tons of USABLE wave types, creative filter types, great sounding effects, and etc. It’s one of those synths that i use as a barometer when deciding whether or not I want to buy another synth, because it can usually easily cover whatever that synth can do. Kind of like a Virus TI2 in that way, but much, much more user friendly.
Best sounding synthesizer: Black Corporation ISE-NIN.
No, it’s not a Jupiter-8, nor does it profess to be (it was inspired by the J-8). But it sounds drop-dead gorgeous. Words cannot describe.
The ones with no screens or tiny ones.
In my case :
Heeeey! This is not fair!
Op has asked for ONE synth!
Everyone can list 20 favorite fun synths!
Most fun: Playing around with the sample engine of the Microfreak
Buchla 208c (+Linnstrument)
Sequential Take 5
Majella Implexus
Digitone
Syntakt
For me it’s the Subharmonicon.
Second place would be Pulsar 23
Most fun: Avalon
Best sounding: Virus Ti
Novation Peak. The sound is huge, digital oscs stay in tune and the front panel is very knobby and hands-on. You can do acid, Vangelis, Aphex Twin, Jan Hammer, Paul van Dyk or whatever, it’s just a really good workhorse.
Sorry, I’m sounding like a broken record as it’s all I talk about here but the OB6 is incredibly fun and playable. I have a Low Frequency Expander for it which gives me 4 more LFOs, an envelope and a dedicated distortion knob (such a mistake that they didn’t make room for one on the actual OB6 as it’s pretty much the only thing behind a menu on the whole box and so fun to play with).
Even without the LFE (Digitakt would do the same thing pretty much) it’s the most beautiful and fun thing to sequence a pattern on and play live to make all kinds of sounds. Having knobs for everything and no menus is the best of user experiences. I never use presets though probably should so just init a patch or go into manual mode and make things. Every day where I feel uninspired or that eveything sounds the same on it leads to me learning something new. The midi implentation is so good (obvs because Dave Smith) that you can add midi lfos to everything. I’ve been doing lots of:
I kinda want two so I can do multilayered stuff but might get something else to pair it with instead. But it has to be something that’s as instantly playable. The Matriarch looks quite fun but the patching puts me off when it probably shouldn’t.
I only wish I had more hands so I could do more with it. Have an expression pedal but may get more.
I love the Hydra but I just used a friends Quantum and wow I thought I would hate it because of the touch screen not such a fan of them…
Turns out it was the most fun synth to program and makes incredible sound! of course the price is incredible also unfortunately but FUN yes it was indeed.
Most fun Synth I actually own is the PRO 3 SE amazing build quality tons of distortion and feedback options pleasure to play and Sequencer is ridiculously capable and fun.
The Polybrute. I love turning it on and going “I wonder what will happen if I do THIS?!”, and then completely losing track of time.
The sequencer and arpeggiator on it is really nice to get some notes going and then jam out with the many controls on the panel.
If you spend some time mapping the touch strip and morphée it’s very tactile as well.
OP-1 is my most fun synth. But only when fooling around. When trying to produce on it it becomes my most hated.
I love to make “things with things”, that’s why I prefer the Hydra with the Mutants and the possibility that the Oscillators can be routed to each other. That’s also why I love Syntakt (routable overdrive, a form of synthesis) and Opsix (routable operators in user algo mode). [Before one says “modular”: I also love Total Recall 
]
I had the Idirium and am happy I could sell it without making a loss (it was a very low price) ^_^;
Your icon is saying nerdseq 
