I had an E-P5. It was quite compact for its days, but moving forward, low-light is piss poor and size-wise, Sony APS-C mirrorless is getting more compact as well (with larger sensor to boot).
I don't think there are any significant improvements in MFT bodies over the past decade, so any that you an afford will be just fine. I would consider Panasonic models though. Not sure about the recent models, buy my Olympus E-P5 suffers from shutter shock, and it can be highly annoying.
Olympus introduced a pre-shutter option (diamond icon on the drive mode) that accompanied every shooting mode (single, burst, self timer, etc). It eliminates shutter shock at zero expense to the shooting experience.
The Olympus also "outshot" the contemporary APS-C and FF cameras I had in low-light because the ultra effective image stabilization system allowed me to shoot multiple stops longer handheld. Its not so much of an advantage now because IBIS are becoming more common...although theyre still not as good as Olympus'