Good evening and welcome. I hate to start by saying "it depends" but the answer depends on two things - what you view as mid-range and whether you prefer (or are more comfortable) with SLR or Rangefinder form factors, and if the latter, do you want direct vision or are you happy with an EVF.
Technically, top of the tree at the moment are the X-Pro1 and the SLR-style X-T1. The former is just about to be replaced so is available new for very few pennies, making it a real bargain. The X-Pro1 has a hybrid viewfinder combining direct vision and EVF in a rangefinder-like package. Both are robust, pro-spec bodies. The X-T1 also boasts weatherproofing, a feature that is expected on the X-Pro2 next year.
The next tier is occupied by the X-E2 and the X-T10. The X-E2 is again rangefinder-style, but only offers an EVF. The X-T10 is relatively new and is SLR-style in it's handling. In the secondhand market you can also find the X-E1.
The most basic models are the X-A1 and X-M1. They have no EVFs and limited user control compared to their bigger brothers.
What, of course, they all do, is take the exceptional range of X-Series lenses, so once you have settled on a body, the world is your optical oyster
Hope this helps.