Socket 1 or Socket 7 seems probable for vintage sockets, if interposers are allowed. If we include modern sockets, AM4 is probably the most upgradable socket ever.
Bristol Ridge "dual"core bulldozer 3.4ghz turbo freq APUs -> 5950x 16core/32thread 4.9GHz boost freq monster.
GHz increase isn't insane but 8x (16x? AMD got in hot water for describing Bulldozer "cores" as real cores, so divide any Bulldozer by 2 to be legally accurate) as many cores on a dramatically better architecture is insane for performance.
Ignoring multicore seems unreasonable, especially for a socket as modern as AM4.
AM2/AM2+/AM3 are worth examining too, but I don't think AM3+ is worth looking at as a result of how miserable Bulldozer is, unless we're working with purely frequency numbers.
775 performance uplifts were a lot for a modern socket, especially on the singlethread, but for multithread they're not as insane as AM4.
Passmark:
Celeron 420 -> Q9650
471 ST 235 MT -> 1292 ST 2411 MT
~2.75x singlethread, ~10.3x multithread
A6-9500E -> 5950x
1478 ST 1841 MT -> 3298 ST 46198 MT (not a typo)
~2.2x singlethread, ~25.1x multithread