I do quite like the III - I wasn't expecting to, to be honest - but the IIIf is the one to go for if you're buying into the screwmount Leica world and want just one body to rule them all. It's a lot quicker to use due to the arrangement of the eyepieces, you get 1/1000 (the III stops at 1/500), click-stopped slow speeds, and fully adjustable flash sync.
(The IIIg is nice too but costs 2-3x the price of a IIIf, and the IIIc loses the flash sync and often has lower-quality chroming.)