A fighter who throughout his career was always game and could back it up with an iron chin, endurance and solid technique that however many fights under his belt, fighters from any generation struggled with him. He may not be the best, but he always brings his A game thst has every fighter guessing and struggling.
By this definition, Jon Fitch. Love him or hate him, virtually every fighter he went up against struggled, win or lose. The only ones who didn't were GSP (the GOAT) and pre-USADA Jonny Hendricks.
Honorable mentions to Cowboy, Shogun, and Randy.