All time is sure tough, but I'll throw in my .02. I'll stick (mostly) to gassers, I am not and never will be a diesel guy. Some of these will be controversial, but here goes:
Buick 231 V6. Yes, it was occasionally plagued with ENGINEERING faults due to cost-cutting, however the basic design is legendary
Obviously, the SBC. Including the 4.3 V6.
LS engines (type dependent)
Buick 215 V8. All aluminum, developed in the 50's, put into production in the 60's. Rights purchased by British Leyland who continued to build and enhance the engine until it's last gasp in 2004. Sure, the Brits ruined it with the electronics however the basic engine is almost impossible to kill.
Most inline 6 engines. Be it the Chevy 325, the Ford 300, the AMC 4.0, the Chrysler slant-6. All amazing feats of engineering.
Ford modular motor, the original 4.6/5.4. Faults? Sure. Reliable with very little maintenance? Absolutely.
Chrysler 318/360
Toyota 5SFE
Toyota 1ZZ
Nissan VQ
Honda J-series (and many others)
Mitsubishi 4G and 6g engines
I'll cut the list there, but I will say the topic brings up a good point: building a reliable engine in the current is much more difficult than it was not even 15 years ago. For example, imagine trying to make the Buick 3800 viable in 2022... it would be impossible. Cast iron? What in the world is that. Pushrods? Those are for pickups. Add all the modern tech to a 3800 and you can kiss reliability goodbye. DI, VVT, AFM, an 8-9-10 speed trans, take your pick. The LIM gasket "issue" is peanuts compared to the stuff I see with modern engines.