I am positive various sweet-spots for certain price/performance, imaging aesthetics, convenience, cost, etc, etc pertaining to photography are different for different people. After running the gamut of formats, manufacturers, options, etc I understand where mine are and am pretty comfortable with them. I figured I would chat about them so you might better understand your own needs and desires when choosing various bits and pieces of kit for your own ends.
First off you really have to know where you live in terms of what kind of pictures you take and just as importantly how you want to take them and ultimately how they actually look. For me I live in "normal" lens land or not too far off. Pretty much 35mm to 85mm. Not a very broad range for sure. That's because I like to make pictures of normal size things from close up. Not macro close but somewhere in the neighborhood of 3-10 feet. I like them to feel like you are close to the subject. That drives my choice of focal length in a major way. Anything over 85mm or sometimes 100-ish starts to feel far too far away like you are looking through a telescope. Anything wider than 35mm starts to scream perspective effect and crazy distortion of proportions to me. I like things that don't look like "lens effects" in terms of telescope or wide-ness. I rather it be pretty hard to tell what focal length I happened to use. I don't like spacial relationships to be too far off from what you actually feel/see when you are standing exactly where I was.
I also tend to live at very similar Apertures in the range of lenses I use in all the conditions of light I use. Somewhere between f/1.4 to f/4. In fact I try to optimize ISO so choices so that's where I live on purpose now. It was born out of necessity if you look at it back when my ISO options were either 100-ish or 400-ish. There's not much you can't shoot in the light I like with that small range. ISO 400 in combination with a normal-ish lens at f/1.4 is pretty damned doable in just about any light. It turns out that I like the way the subjects I gravitate to look at those apertures - specifically at the distances I happen to shoot at - even more specifically with the size of the capture medium I happen to use… 35mm sized image capture medium.
Now for convenience. In the film era 35mm devices were as small and most of the time actually smaller and more convenient than even APS-C sized digital devices even with viewfinders which a lot of digital lacks in APS-C. Turns out APS-C is absolutely the smallest capture medium I can and will live with - even that's a compromise because of a few factors. The most important being that lens choices are barely equal in terms of maximum aperture at the focal lengths I want. They actually need to be a lot larger to render the way I like my images to look. Forget 4/3 - just looks all wrong. So 35mm or full-frame or FX whatever you want to call it is a natural sweet spot for me under a very wide degree of circumstances at reasonable ISO's with a stretch sometimes to 1600 - that's like 6400 if you happen to be using a f/1.4 vs a f/2.8.
I can get the same look from traditional medium format with a similar range of lenses say 50mm to 150mm. Here's the rub - even though the look can be similar or even more so "better" of what I am looking for the actual maximum apertures are slower in that range - in the best case they are 1 whole stop slower and that's only available in two mainstream lenses ever. One from Hasselblad and one from Contax/Zeiss. To shoot them the cost is extraordinary - even today. In most cases it's more like 3 stops slower. The range of lighting conditions is very narrow comparatively and the size goes through the roof. So traditionally they have served the role of "special purpose" - good light cameras. Even now. Yes 35mm is my sweet spot for a capture medium size. Even the physics of enlargement to final display sizes has proven that to be the case for the vast majority of my display purposes - 6x9 or smaller.
If your preferred subjects happen to require long lenses APS-C or 4/3 will probably make either no difference or be far better for you as long as you can get the IQ from whatever technology exists at the moment. Your images will look pretty much the same. There is no DOF to speak of anyway. Whatever is behind your suject when using long lenses will be completely obliterated in any case. It may be exactly what you wanted - you may actually really want 12" of DOF vs 3". Same goes if you are someone that needs or wants boat-loads of near to far in focus subject matter. Smaller formats may be better and provide you with exactly what you are looking for.
For my purposes 35mm is a fantastic general purpose capture medium. It'w why I complain so very bitterly that the best digital option for me at the moment is the Nikon D600. It's still laking on things I value for the imaging characteristics I desire. It's still far too big compared to my really old cameras that deliver the same imaging characteristics I am looking for. The viewfinder while comparatively decent is still not on par with a lot of my older cameras. At least it delivers pictures that look about right. I hope to see further reductions in size but I fear even if I do - the lens trends have been to be bigger and bigger and bigger to house the focus motors etc.
As for smaller formats somehow getting the maximum aperture to deliver similar aesthetics - fat chance. The lenses would have to be HUGE which defeats the whole point. Even my Fuji X100S is not exactly what I really want. I support it because it's the best to date in what I want in a camera but it's actually got a pretty big gap for my narrow range of desires. Specifically it would have to stay the same size - exactly. Acquire a 35mm sized sensor and get a 35mm lens of f/2 - preferably f/1.4 but I could live with f/2 on full-frame. Then it would deliver images a whole lot like any of a dozen 70's era consumer japanese range-finder cameras. Funny part is it would still be bigger.
The image at the top is my X100S vs my OM-2 SLR. Yea I have told you the XPRO-1 was larger in every way than a lot of my film DSLR's - what I didn't mention is that so is the X100S - well a little bigger than my smallest SLR and about the same as a lot of my other favorites.