I'll give you some short takes, since I've used all of these cameras.
GS645 - fragile, marginally smaller than a GA645; Fuji's only camera with a 50mm equiv. Filters are a total PITA.
GA645/i/w/wi - pretty much the value leaders. Best viewfinders.
GA645zi - a beast of the camera that takes phenomenal pictures. Worst viewfinder.
G690 - first interchangeable-lens model. No dark slide interlock.
- Lenses: chrome 65/8, chrome 100/3.5, chrome 180/5.6
G690BL - second interchangeable-lens model. Adds a dark slide indicator in the VF.
- Lenses: black 65/8, black 100/3.5, black 150/5.6, black 180/5.6
GL690/GM670 - third interchangeable-lens model. Removes the dark slide indicator, changes the loading slightly (red dot position is different). Takes Nikon eyepieces. Ugly new hump on the lens mount. Front shutter release added.
- Lenses: black 50mm f/5.6 (new) black 65/5.6 (new), black 100/3.5 (now with colored DOF scale), black 100/3.5 AE (new; only lens with MC), 150/5.6 (unchanged), 180/5.6 (unchanged).
GW690/670 - essentially a lighter version of the GM/GL with no lens interchangeability. Some parts are now plastic (which unlike the previous models does not dent and scrape so badly). Five-element 90/3.5 (~37mm) replaces four-element 100/3.5, increasing sharpness and degrading OOF areas slightly. No AE options Shot counter introduced. GSW versions use derivative of 65mm f/5.6 Fujinon (~28mm) released with the GL/GM. All versions now are in permanent "R" mode, which prevents shutter firing without film.
GW690/670 mark II - hot shoe added. Somewhat annoying sliding hood added (it obscures the exposure controls).
GW690/GW670/GW680/GSW680/GSW690 Mark III - obscure 6x8 format resurrected. Much more ergonomic bodies in an easy-to-damage matte dark grey plastic. GSW finders updated to have hard-edged RF spots. Bubble level added to top of camera.
On fixed-lens 6x9 cameras, I find the 90 to be the most useful generally. The 65 is kind of the odd bird: wide enough to be hard to level, and if you add an accessory finder with a level, what was the point of the superb built-in finder?
I am actually a big fan of the GL with the 50mm; the 100mm is the second-best choice. The 180 is pretty much a bench warmer.
Dante