If I only had one camera, I'd probably make it the Olympus E-PL5 at this point.
I have two Micro Four Thirds cameras right now. The Panasonic GH2, and the Olympus E-PL1. I primarily bought the E-PL1 to use telephoto lenses on while I use my usual landscape wides with the GH2.
I love the GH2. It has, in my opinion, the best control layout of any small DSLR-styled camera to date. And it was the first move forward from the 12.1 megapixel sensor first used in 2008's Olympus E-30 that was also capable of autofocusing most Four Thirds lenses.
However, since I got one, the camera I've nearly always taken with me when I just want to grab a camera and go has been the E-PL1. There's really not much size difference between it and the GH2, (the GH2 just has a beefier grip). They both focus fast enough, and pretty reliably in poor light (with an edge in both categories going to the GH2). But what clinches it in the E-PL1's favor is image stabilization. I feel as though the much better high ISO ability of the GH2 is not as good for handheld shots of static subjects in dim light as the E-PL1's image stabilization is, and most m4/3 lenses do not have image stabilization (to date, only one of my lenses does, the 14-42, and I'm retiring that lens in favor of the Sigma 19mm and 30mm).
The GH2 still gives me far better files than the E-PL1, making it remain my primary camera (particularly since the majority of my shooting is on tripod). The bulkier form factor makes it easier to use in the outdoors. However, if I were to upgrade to one of the newer Olympus cameras like, say, the E-PL5, that would probably become my go-to camera for all things, and it would be my most likely upgrade path given what is currently available.
