I agree, every combination of turns is a little adventure.
Most fun for me are monomachine and Kurzweil k2000. They can both be taken to interesting new places.
Most fun I’ve ever had on a synth though was when I first got a microkorg.
I´m a long time Rytm user but definitely need to try a Syntakt. Do you consider it much overlap with the RYTM?
Hi! Rytm and Syntakt sound completely different and are almost incomparable, even if some of the machines bear the same name. My favourite is Syntakt, but I wouldn’t ever part with my Rytm either 
Edit: I wrote a little bit more here Syntakt, the most complete drum machine? - #10 by Jeanne
Behringer Edge and Syntakt
Great to know… thanks for the opinions.
Most fun Synth ever is the Octatrack
Just to add a couple of cents on the original post…
Over the years, my feeling of what is fun has changed a lot. I used to like playing deep instruments (Nord G2, Kurzweil K series…) and tailor-program them. But at some point, I got tired of patching and not making music, read chorus and verses 
I took a curve towards simpler instruments and found out that less options suit me better. Modular? Forget about me. Limitations? Yes, they either challenge or channel creativity
The question all this raised for me was: what exactly is fun?
I only knew that -to me- fun is not proportional to the amount of options.
I also knew that it was lying somewhere near the user interface: I had an Evolver which interface was smart and immediate… but I kind of replaced it with a Little Phatty because -despite the few knobs- it was way funnier to me to use.
Later on, I wondered: is fun related to the fact you can get lost? Well, I like the feeling of “being understood” by the instrument… So I’m more at ease when I know I’m a couple of knob turns away from what I want to achieve. For that reason, again, simple is beautiful to me… but also the way commands react: the range of each command can make a great difference at equal function set.
Example? The Vermona synths I’ve tried sound great, but the parameter range make them less enjoyable to me than any Moog synth. Obviously, that make Vermonas more versatile than Moogs at equal function set. However the simplest Moog will definitely lead to some happy accidents. Happy accidents are fun.
Bottom line: fun is something moving that I know I will chase all my life and I’m ok with that 
Now, if I have to answer simply:
I have had great fun experiences playing a Little Phatty for years now. Maybe more than with a Voyager. The MG1 is great fun too. MDUW is also in the front runners, but I get more “cerebral” on it. Cerebral is not fun to me. Oh, and the Septavox.
Best sounding instrument… even more difficult one. Moog synths for sure. Let me mention Precision basses too!
Microfreak, easily. I make at least one patch every time I use it, and it’s just a blast to use.
Another vote for Microfreak, so much power in a small package, yet accessible. And if we talk fun per euro spent….
I got drunk one night with my Norand Mono and we ended up doing karaoke at a 4am bar, so that’s my pick.
Most fun: TD-3 triggered by TR-8s (quick, immediate and great sounding (acid) fun), Yamaha FGDP-30 (turn on, select drum kit and play - no cables and no setup required)
Best sounding: Opsix (eargasm), TR-8s (much more alive than using samples of those classics)
Ironically, those are some of my least used devices (not even a part of my main setup), but I keep them specifically for their sound and/or fun factor.
I only own and have owned a few. I think I would say my Vermona Mono Lancet, its such a great sound in a nice small package with very limited but accessible and intuitive controls.
This thread makes me want a Syntakt again among others (OB-6 and more…sigh)
most fun i think i’ve had with a synth was the Moog Prodigy.
right now I think my Analog Keys is the most fun but maybe my Syntakt is…it’s a toss up.
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I have most fun when using a single groovebox like Deluge, Syntakt, SH-4D, EMX-1. No cables, no setup, no syncing issues, etc. Just sit down and enjoy jamming out.
Best sounding synth would be the Waldorf Iridium. I’ve not owned one but I tried it in the store and it sounded amazing. Way above my budget though.
Syntakt for me too. It’s the only groovebox I’ve felt is both intuitive and full of untapped possibilities. My “fun” setup is the aforementioned Syntakt with a smallish modular and Toraiz Squid for creative sequencing.
Any synth that I can lose myself in is inherently fun. The conventional synth that does that for me is probably the Nord Lead A1 as it is so intuitive. Just wish it had a better keyed. The modular is the other place I can get completely lost for hours in hands-on experimentation.
Drambo. Because I can sequence, layer, process or resample an endless number of synths on my iPad from AniMoog to Twin 3.
Hardware wise, easily Roland Aerophone. As a sax player, I can do expressive things with a Zencore powered wind synth no other synth including MPe ones can even approach doing.
Korg Kronos with a 1010 Blackbox is my goto fun combo for music making and practice.
I used to do all of this directly on the Kronos. However, generating audio stems from MIDI tracks on the Kronos (“bounce to audio”) is subject to unpredictable MIDI clock/timing related errors during export, making this crucial task both unreliable and very inconvenient. Since bringing in the Blackbox, the music making process has become fun again.
Fun: prophet 6 desktop. Limited in such a nice way. Really hard to mess up a sound
Best sounding: eurorack modular
Difference between listening to sound and be totally drawn in. Hard to explain or capture
Fun = Analog Rytm MKII