I think mine would be the OG Saucony freedom.
While some people have complained that they are a little tight and unstable, you can actually remove either the insole, the EVERRUN topsole, or both, to get 4 very different-feeling shoes. Leave both in for a highly cushioned lightweight trainer, or take out the insole (running directly on the EVERRUN topsole) for a midweight racer with fantastic energy return (that also fits more true-to-size, imho).
That isn't to say the SF is my favorite shoe; I just think it is the most versatile. Second in terms of versatility would be either the Altra E Racer or the Skechers GoRun Ride 7, but the Racer possibly just a little too minimal for long training, and the GRR7 is a little heavier than I would like (plus the FlightGen midsole, while possibly the best EVA midsole out there, is still a small notch below the TPU midsoles of the Racer and Freedom). The NB Beacon would also be in the conversation for 2nd place.
How about you? You only get to pick one shoe for all of your training and racing for the next, say, year and a half. What shoe do you pick, and why?