Rrryan
63
Hmmm.......
Been thinking it over, and it's not easy to decide. What occurs to me is that Omega's evolutions seemed to occur in phases for which dividing by decades seems somewhat arbitrary; in some cases lasting developments were years apart, in others major developments occurred within a few years of each other. But for the sake of discussion I'll give it a shot:
1940s: 30T2 series is a very good choice. Sturdy, reliable, award-winningly accurate performer used almost unmodified until 1963. Arguably the most significant development in Omega's history besides the actual "Omega" pocketwatch calibers of the 1890s.
-Runner-up: Seamaster, 1948. Capable movements in watertight cases, which gradually were adopted across production
1950s: Tough to choose. If just 1... probably the Constellation. It took advantage of Omega's high-end calibers and became the famously well-regarded flagship line.
- Runner up: almost any of the bumper and full-rotor automatics
1960s: Pretty easy choice: Speedmaster Professional. Though the Speedmaster was introduced in 1957, it didn't become so famous until 1965 when it became the "Professional" following NASA adoption.
-Runner up: the f300 tuning fork models. A harbinger that mechanical technology was near obsolescence, but simultaneously in which Omega asserted its technical and research prowess
1970s: Hmmm.... not too many choices here. Perhaps the 10xx and 11xx high-beat but low-finish calibers are most characteristic of the direction Omega was taking: downward
1980s: I can't really think of a good choice here. Maybe the Titane sports models that showed emerging case and bracelet designs. But on the other hand, the most significant thing in Omega's history at this point was arguably not an Omega at all but the Swatch. There were plenty of unique technical developments, but most were more novelty than significant.
1990s: I think the "Bond" Seamaster of 1993 is a good choice for this era as in many ways it signified that Omega was "back" after years and years of playing catch-up to the Japanese and emulating Rolex designs. It was a genuinely unique and original design with plenty of merit in its own right, sold in droves, and it didn't look like a Rolex. It also served as the basis for many later designs. While technically the De Ville Co-Axial was released in low numbers starting in 1999, the co-axial developments belong mostly to the 2000s IMHO despite Daniels' having invented it decades previously and its 1999 introduction.
2000s: Agree it's hard to say yet. The Co-Axials (especially the 2500C in which they finally evolved into maturity) would be a good choice, as well as the semi-in-house FP-based chronos. And it's hard to say yet how their significance will compare with the new "in-house" calibers. Ask me again in 20 years. 
Regards,
-r
