All the cable discs I’ve had personal experience of (never tried a set of BB7s) have been only on a par with, or sometimes inferior to, a well set up V-brake.
When you say ‘canti brakes’, to you mean pre-V cantilevers, with a central staddle yoke, or do you mean V-brakes? If the former (and you rate those as 1), then I would put V-brakes at 6 and cable discs at 4 or 5.
Of course, you do get the ancilliary benefits of discs over rim brakes, such as a buckled wheel having little effect on braking and the braking surface being more out of the way of flying mud. With hydraulic discs starting as affordably as they do now, however, I can’t really see an argument for buying cable discs (unless of course you are using them on a bike with drop bars!).