The king of overclockers was the 286. Implying the internal clock multiplier somehow makes it more overclocker friendly is pretty stupid, as it's got nothing to do with anything.
That was made popular by the first internal cache on x86 processors. Overclocking a chip without an internal cache had poor performance improvement. But, again, that doesn't relate to overclocking. It does relate to the fact that they were reaching a point where it was very difficult to make motherboards handle bus speeds as high as the processors (although Intel would return to that with the original Pentiums, never to return).
The 286 was a more complete overclock, since it increased the speed of everything. If you overclocked the 286 by 33%, everything ran 33% faster, even the system bus. That never happened again. Most implementations of the 386 (and even later releases of the 286) ran the AT-Bus (commonly erroneously called the ISA bus), at 8 or 10 MHz, regardless of CPU speed.
Overclocking the original PC/AT 139 was so common (from 6 Mhz to 8 Mhz), IBM put a timing loop in ROM for the 239 that prevented it. Eventually, the 339 ran at 8 MHz out of the box.
Overclocking these machines often required more skill than modern machines, as you'd have to buy a different crystal, desolder the old one, and solder the new one in. If the memory chips were close to timing, they'd have to come out too. Luckily, most were not soldered in. IBM's had a perverse piggybacked arrangement though, where they would put one 64K chip right on top of another, and they were soldered together. It looked horrible, but somehow worked like a 128Kb chip (which there never was, they went from 16K to 64K to 256K).
Leaving out the 286 is silly. It was really the first one commonly overclocked (I never hard of anyone overclocking an 8088, but someone probably did), and actually forced the largest computer company in the world to put in safeguards around it to prevent it. As strange as it sounds, my original 8 MHz Tandy 3000 showed a nice improvement going to 10 MHz. One SCSI card didn't like the faster bus, but everything else was fine. Adding in a zero wait state memory card from Cheetah also dramatically improved performance.
Even the 386 was commonly overclocked, and there were a lot of scandals because companies would buy lower clocked 386s from Intel, get rid of the silk screen, and than overclock them in the machines they would sell, without telling anyone. What a mess that was ...
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