Thinking of something narrower: Panasonic 42.5 F/1.7, Olympus 45mm F/1.8 or Sigma 30mm F/2.8

I own both of the Panny 42.5 lenses.  Bought the f1.2 when it first came out.  When I bought a GM5 to be my "never without it" camera, I wanted a three lens kit that would be really compact, but still good in low light.  The Oly 12mm f2, Panny 15mm f1.7, 20mm f1.7, and for telephoto... which is what led me to buy the 42.5 f1.7.

No question that the f1.2 is an amazing lens.  I posted some shots on one of the "this week" threads, portraits and action shots of my dogs romping and wrestling (breaking in a GX8), and you can see how thin the focus is.  If the eyes are in focus, the nose is not, nor is the neck fur.  What you can't see (because I didn't post them) are the misses, where because the DOF is so thin, the camera locked on the nose (so the eyes aren't in focus) or the neck fur (where they both have a high contrast light/dark blaze) so nothing forward of the neck is in focus.  The lens focuses reasonably fast, but not as fast as many other lenses.  The amount of glass that the focus system moves is enough to slow things a bit.

The f1.7 is also amazing, and in many situations may be the better choice for people.  That one additional stop means that if the focus locks on the nose, I'll still get the eyes in focus.  Yes, I could just set the f1.2 to f1.8, but there'd still be the issue of focus speed, size, and weight.  The f1.7 is so small and light, it's impossible to imagine NOT taking it with the kit, where with the f1.2, I can't take it with the lens variety I want in my smallest camera bag.

I think both lenses are very high quality from an image perspective.

If I'd have bought the f1.7 first, I probably wouldn't have bought the f1.2.  In particular, the f1.2 on a GX7 body felt odd, kind of unbalanced.  With the G6, less so, but still it felt like the lens and the camera were from different systems.  On a GX8, the f1.2 looks exactly right.  Putting the f1.7 on the GX8, it looks oddly small, but that lens is PERFECT proportionally on a GX7 or GM5.

I don't think you can go wrong whichever you pick.  Worth visiting a camera shop with the body you'll be using to check out the size and feel differences.  Easy to say it doesn't matter, but it does.