It depends. For action, it's my K-5. For wildlife, my G5 with 100-300mm zoom. But all around right now it's probably my E-PM2. I just ordered an LX7. It'll be interesting to see where that will fall on my preference list because, despite owning a number of cameras, my go-to camera for a long time was my LX3.
I'm a filmmaker-wannabe who came to still photography from the filmmaking-cinematography side of things (VCA grad). Hence the video function is important to me. I own a Red Scarlet but that's definitely not a carry-everywhere camera.
I love the M9 for it's simplicity. Nothing fancy or complicated, it just takes pictures. Expensive, well, yes. I gave up a bad habit a little over a year ago and with the money I saved I rewarded myself a used M9. It goes everywhere with me.
If I had to keep only one, it would be the E-M5 for it's versatility and excellent IQ. But I love the X100, and I think the hybrid viewfinder is brilliantly realized. I did not realize how much I missed an optical finder until using it. Now, if it weren't for my investment in m4/3 lenses, the versatility of the system, etc. I might well jump ship to an X-Pro 1.
But I do love the controls and shooting with the E-M5.
^ in that case, may I suggest my old beaten up Nikon L16? The noise it makes when extending the lens is quite possibly loud enough to hear on the next continent... would make for an interesting way of signalling the old SOS morse code!