My vote is for the Visoflex II/III - amazing piece of work. You can mount just about anything on it - and if you get the bellows II - even more stuff. I frequently use enlarging lenses for extreme close-ups, though the "exposure" math can be a bit "taxing" on the brain (bracket like hell and keep notes)
There are enough adapters, rings, helicoils available to keep you entertained for years too! They are not expensive - for what they are. Today it would be a $1500-$2000 thing to make - and you can usually pick up a Viso II/III and bellows for $3-400. The rings usually run around $15-25 for each.
I have it and use it frequently - but for ease of use, nothing beats an old Nikon F and the 55f3.5 Micro Nikkor (and I have to admit, also the 105f4 Macro, the F-Bellows with the 135f4 and a 200f4 Micro - oh well, they were cheap - all of them less than a beater M2 would cost!).
The Elmar 65f3.5 is very good, almost rivals the 55f3.5 Micro Nikkor.) In chrome they are $4-500, the black ones are coniderably more. Supposedly it is slightly better - I have the black version and used to have the chrome too - not that much difference.
The Dual Range is good, but with clean glass and correct goggles - they are not cheap (easy $5-600). If you are going this way, try to get as high # as you can find - 2000 000+ as Leica fiddled with coating etc on that lens and improved it as they went along.
Heavy though - and limited range. Looks great too - and it is very good, sharp, medium contrast and you spend a lot of time swaying back and forth trying to pinpoint that minimum distance focus!