I'd wager either the Olympus Air or the Z Cam E1. There might be another boxy camera like the E1 out there. Perhaps it was a Kickstarter project, but I'm forgetting.
If you're considering purchasing either, I'd like to share a few thoughts. I didn't end up getting the Air because every review says the performance of use is bad. It has a hard time staying connected to your phone, which is required unless you wish to shoot blind. The, I think it's Bluetooth, was really buggy, and it never got a patch. It's a shame it didn't. The design idea of this camera is very nice. Think Hasselblad 907x, but for m43. The thing really had potential.
The Z cam...it's ok. As a stills camera, its performance is very weak (one big center focus area, terrible af that's slow), but the images are acceptable enough when taken properly. Video, no matter what the company's official line is, should not be shot with their log profile. It's 8bit and falls apart fast. The regular 4k, from my use, was acceptable enough when shot right, but I've seen people make that footage shine in ways I never could. For under $250, it's a fun little lark and can be "hacked" for "better" video. The people asking $400+ can keep them as far as I'm concerned. When they stopped making them, they went on sale for $250.
If you're searching for some combination of small size, quality, and performance, along with some powerful hacked video options, I'd recommend an EOS-M (the first model) far more. It cost about the same on the used market as either the Air or the E1, and it just flat out a better camera than both in so many ways It's ridiculous.
If you're only going to count mounting m43 lenses....I'd then throw the hat towards a hacked Insta360 One with the bareback lens mount adapter. The price was $1500 for that last I knew.
