I dunno ... I had a 4-speed truck box last in a 1962 F-100 trough three engines (close to 500,000 miles) and it's still running fine w/o any issues in the restored truck it's in now (across town). I don't know of to many autos that will go 1/2 a million miles... Maybe a Allison in bus or something ...
I do know of a number of RTO125-13 truck trannies that went at least 1.5 million miles before having bearing replacements.
But, I guess you'all are talking cars and light trucks
In that case I'd vote for a the mid-1960's Ford C5AZ all synchro iron case three speed. It was std issue in pickups and cars with anything from 250 sixes to 390 V8's and was pretty much indestructible.
We had one in a Dirt Track car. Ran 1/4 mile tracks in second, and longer (faster) tracks in 3rd. Saved having to change rear-end gears
And we were feeding it prodigious power and beating on it all the time and it never whimpered or broke
On the street I ran those with a Fenton 500 shifter behind a bunch of big healthy Fords (406 ~ 430 inches) in the play cars (mostly 2-door Ranch Wagons). I'd side step the clutch at 3,500 and leave hard time and time again all night long down on 1st & 3nd Streets in San Jose cruising and street racing. Same on Beach Street in Santa Cruz or out at 4-lanes on the coast highway.
Only Auto that ever came close was the 727 TorqueFlite. You could neutral start that tranny behind a big block and it would leave hard too. Then guys figured out how to build converters and that era was over.
Either tranny would last longer than OEM rear-ends
But, as stated above, now-days it's real hard to beat a TH-400. There are some beefed examples behind 600 inch blown outlaw motors running E-85 making 2,500 HP on the "street" and running in the 8's. They hold up well enough
the next step up from there in a Lenco