My vote goes to the Diamond "DFM UMP".
Has to be the lightest I ever held!
I retired my +POS for a "Pro-Nine" which I later gave to the Mrs to retire her +POS. Those were light, but the visibility in the +POS is not real good (not that I can see anyway, just ask the fans!). When I picked up the DFM-UMP in the store, I knew I had to have it.
There are a couple of masks reported to be the lightest available, not sure which one really is, but the Diamond feels lighter than the Wilson, and the All-Star.
EDIT....
Per this site,
http://www.theofficialcall.com/Diamond%20Ump%20Mask.htm
this mask is 18.1 oz. I'm not endorsing the site, don't know 'em, but the price sure ain't bad!