The Microfreak is a lot deeper than it seems at first. It does only have a few knobs for tweaking the various synths, but the magic is in the modulation matrix, and all the knobs for everything else. Plus the great analog filter.
Also, the keyboard is really nice, with aftertouch (can’t remember if it is poly or channel aftertouch).
The only downside for me is that it doesn’t have reverb or delay. And you need to slap a magnetic battery pack on there for true portability.
neither do i but you said you wanted deepest synth(s) 
I myself have a history of compromising synth depth for other priorities, like tactile experience, ease of use, etc.
We have several people here who have been looking for years for a superpowerful synth that can fit in one’s pocket yet at the same time be easy to use and have a pleasant tactile experience. Various solutions have been proposed but all have some compromise or another.
When a synth is too small, people start to complain about the display being too small, knobs/faders/whatever too small, not enough knobs/faders/buttons, etc.
Life as an electronic muso is full of compromises
MC-101, if you don’t mind menu diving.
Or maybe Polyend Synth.
I don’t know of anything truly portable that comes close to the sound design potential of M8 (maybe MM2, but even that is restricted to VA, albeit very good VA). Since you own M8 already, it might be worth seeing how the knobs and pads on Bento can be used to improve the M8 experience for you.
+1 for the DNII
If I had to take one synth to a desert island, it would be DN2, but it’s a little too big and heavy for me to casually take on a trip. I think it depends on the exact parameters of “portable”, which are probably affected by lots of other factors, including various capabilities and usabilities.
yeah, you might be right. I will have to play around!
the opxy is almost perfect! Truly love everything about it besides fairly limited synthesis parameters. If someone paired roughly that form factor with a real synth engine id be in heaven 
hm ill have to read the manual and see what i might be missing.
Cost was a barrier for me against getting opxy.
I have an M8 like you but I got more interested in chords and chord entry on M8 has always been an issue for me. Yes I studied the videos on how to do M8 chords, and even programmed in some songs that way, but I got tired of it.
Have you tried sequencing M8 from opxy?
I’ve traveled before with M8, MC-101, IOS devices… I rarely did anything productive musically. When I’m on the plane I end up taking a nap or just feeling too tired to make music. When I arrive at my destination, my time and energy is occupied by whatever reason I traveled to that destination in the first place.
So now I don’t even try to do “portable music” anymore. I’m at my most productive when i’m at home with my computer with Ableton, and various hardware gear.
yeah, part of me feels like i just need to give up and use a laptop.
Ive played instruments my whole life though, so I really enjoy manipulating controls, and “playing” something. Not to mention what a drag it is to be constantly connecting/disconnecting and finding cables for everythinf. But alas.
The comment above about using the m8 also made
me think maybe I try and control the iphone/ipad from the bento and see how that goes…
I just got my hands on MicroMonsta 2 and it’s amazingly deep but easy to use. You can combine subtractive, fm and wavetable synthesis with loads of modulation, great filters. Feels like you could never get bored coming up with sounds on this thing and it’s so small!
i only saw youtube videos so far, but it does sound really good! Can you import user wavetables? And is the “fm” just predefined fm sounds? was a little confused in the videos.
Fixed wavetables as far as I know and only 2op fm, but the modulators can be any oscillator/wavetable type. combining all these techniques gives nice deep synthesis. Oh and lush reverb…
bento you say?
i was considering getting it specifically for its granular and wavetable synths as a “portable sound design device”. that what i’d suggest in the thread if i didn’t read your post fully
Iridium core? Small and powerful.
Analog 4 
Modal Electronics Carbon 8M.
Waldorf Blofeld Desktop.
Another vote for MC101. Also don’t forget you can link tracks on the OP-XY too. The synth engines aren[‘t deep, but if you’re using 8 at once you can get a lot more depth.
Mc101 is very good, pretty much full access to zencore engine. You just can’t properly save your patches is the problem