"Realism" is an inappropriate thing to seek in professional wrestling as the sport hasn't looked anything close to "realistic" in the best part of a century.
If wrestling was designed to resemble realistic combat, it'd look like simulated mixed martial arts. The great Japanese shoot-style promotions of the '80s and '90s comprised the last scene to attempt this on a macro level, crafting a more forceful, authentic style blurring the lines between pro wrestling and the emerging MMA purveyed primarily by Pancrase, but it didn't last. UWFi, RINGS, and Battlarts died. Long-term, there wasn't a market for what they were selling.
"Believability" makes more sense than "realism," particularly today, and stiffness is a key component of this. While wrestling should ultimately be more about giving the impression something hurts than it actually hurting, executing moves, strikes, and holds with greater force, snap, and pop is often easier to buy into as part of a believable combat exhibition than working loose.
This list won't include wrestlers like Goldberg or Mabel, who were stiff on account of their greenness, often making them sloppy and, in some cases, dangerous. Similarly, wrestlers must have spent a decent period of their career working for WWE, even if their stiffest work was done elsewhere. Sorry, Stan Hansen.
Let's slap some beef...
10. William Regal
Contrary to the besmirched bumbling Englishman he once played on WWE television, William Regal was a rugged, hard-nosed worker in his prime, imparting levels of believability into his own work even when the product around him was largely cartoonish.
"I hit people very hard in very safe places," said Regal of his style on an old episode of Tough Enough. No wrestler has ever captured the duality of stiffness so succinctly. Chops striking a bare chest make an incredible noise and red, welted skin is a ghoulish visual, but deal little-to-no lasting damage. A hard forearm to the side of the neck is preferable to one to the skull. Bruises fade, broken skin heals, and these techniques give the impression of severe punishment when the impact is typically superficial.
Such was the genius of William Regal, the wrestler, whose ugly, ugly wars with Finlay in 1996 are amongst the stiffest American wrestling has ever seen. Even in WWE, where working softer is more prevalent, Regal still found ways to lay it in when paired with fellow hard in-ring hitters like Chris Benoit.