I've been fortunate enough to have fired every one in this particular poll. They all have their high and low points and it should be noted that "workhorse"means different things to different people. To me when I hear "workhorse" I think of it as "If I had to pick one pistol to shoot the living crap out of with the least amount of issues, parts breakages, and intensive service, which pistol would it be?"
While any one of them would be a good choice for a general use pistol, I think the USP and the Gen. 3 Glock stand above the rest from that particular perspective mentioned above. I have not shot any one of them beyond their expected service life, but from the long term data brought in by shooting academies and trainers all around the country, the Gen 3. Glock and contemporary HK polymer pistols are the ones which continue to deliver, through tens of thousands of rounds, with a minimal amount of attention.
When it comes to my interpretation of "workhorse pistol" I don't care what some International Military branch or Police Department issues; they have made their choice on some combination of regional politics, reliability, user-friendliness (train-ability), armorer support/logistics, holster availability, and PRICE. Ouit of all of those considerations, the only one I'm concerned with is reliability; I also consider my own preference for grip angle/shootability, safety features & operation, established track record for long-term reliability, and accuracy in my own hands.