I've got 4 fully functional treadles right now and two under repair. Plus an extra 66-1 head with no home. Two are Singer 66s, one is a Singer 9W-7, and one is a Franklin 1911 which is a copy of the Singer 27. I'm kind of partial to the Singer 66s though. Big arch, easy to treadle and they sew very nice stitches. If I were going to set up one for my only treadle I'd use a 66-16 or -18 so I could have the back tack on it.
I don't know what to say about a ZZ machine because I have no experience with treadling them. I've read that the more mechanics you have inside the machine the more effort it takes to work the treadle. I'll have to put one of my complicated ZZ machines in a treadle and find out.
Joe