This is a question of honest ignorance, but is the Buck 110 a completely original design? Obviously it is one of the most copied designs, but can its true roots be traced back even further? I find it very possible that somebody came up with that style/design first, but Buck mass produced it and made it their own.
I wouldn't be surprised either way. In any case, I'd say that this design is the most copied. Sure, the SAK has been copied, but apart from the 2 true SAKs (Wegner, Victorinox), they're all cheap Chinese made imitations. Imitations of the Buck 110 style can be found not only in cheapies, but in other quality production knives as well, and even customs.
I wouldn't say the Bowie knife is the most copied, since what we consider a "bowie" style knife often doesn't resemble Jim Bowie's actual design.