Depends on how picky you want to be about the definition of 'failure' - a couple of my guns had minor issues with short-cycling etc. in their first 100 rounds, but none since, which could be operator error or just break-in periods.
So going down the list:
FN Hi-Power - ~3000 rounds over 8 years, no failures at all
Kimber Stainless Series II - ~1000 rounds over 5 years, no failures, never cleaned, just occasionally lubed
Springfield XD - ~500 rounds over 5 years, no failures, some surface rust (!?!?) despite the extremely dry central TX climate
Springfield 1911 - ~1000 rounds over 8 years, some mag-related failures with Chip McCormick shooting star magazines. No issues with factory mag or with wilson combat elite magazines
Ruger MkII Target - ~1000 rounds over 10 years, numerous problems out of the box. Several new parts from Ruger fixed it and have had no issues since replacing recoil spring, extractor, extractor spring, guide rod etc.
Ruger Security Six - 300 rounds over 7 years - catastrophic failure in the first 50 rounds when the ejector rod backed out of the cylinder and bound with the frame. Compounded by me brute forcing the cylinder open which bent the crane - had to send it back to Ruger for repairs at my expense. Apparently the prior owner had fully stripped it for cleaning and neglected to loc-tite the ejector rod threads :banghead:
S&W 629 - only owned for 6 months and only ~100 rounds so far, but no issues
Sig P238 - ~500 rounds over 2 years, several short-cycles / FTFs in the first 50 rounds, but no issues since then. Seems to require a higher level of cleanliness/lubrication than most of my guns.
STI Spartan IV - 200 rounds so far (only owned a week
) and no issues.
Guns I used to own:
Charles Daly 4" 1911 - never made it through a mag without some kind of issue. Sold it for a loss and moved on.
Argentinian 'FM' Hi-Power - occasional FTE issue that I could never solve, despite replacing the extractor etc. Sold it and bought the real thing.
Kel-Tec P11 - no reliability issues, but the most unpleasant gun I have ever shot. Only needed to put 100 rounds through it to decide to get rid of it
Ruger P89 - my first handgun, and the only one I regret selling. Put 2000+ rounds through it in a couple of years, and never had any issues
So to recap - looks like the FN Hi-Power holds the record for longest streak. Oddly, my Rugers have been the least reliable (of the guns I still own) which surprised me.