For me a toss up between APS 26240 and ZF RM055. Argugably both groundbreaking.
The ZF RM055 is truely a marvel of case construction, multiple materials used in a singular watch, and that beautiful RMUL clone, maybe not for everyone but objectively impressive. Titanium midcase, rubberized outershell...... not easy to do.
The APS : flyback chronograph. What else needed to say? Even 5 years ago this was unimaginable. Purely the movement alone deserves applause. The APS is a little weak in the dial department with some minor flaw on subdials (but still impressive), and noone has done AP tapiesserie 100% yet.
Both geninuely are marvels of engineering. But for me it went to the APS as the first ever flyback chronograph cloned ever. The RM movement was floating around before hand as hyper expensive custom movements. Even though the RM055 might be a tad more well replicated, my heart says the 26240.