The completely one-sided fight with O'Hara that ended in brutal revenge.
Lee: Boards... Don't hit back.
During said fight, Ohara picked up a couple of bottles and broke 'em, followed by Lee kicking them away. Lee wanted this to look as real as possible and insisted that they use real broken bottles instead of spun sugar. He gashed the living hell out of his ankle and shooting had to stop for several days while he healed.
Releasing the cobra on the guardsmen and sitting there with a bored expression on his face as they panic & flee. That's really Lee's foot you see keeping it at bay afterwards.
The fight sequence throughout the base after the alarm goes off is arguably the greatest fight in the history of Bruce Lee movies. It was further punctuated by a slow-motion scene, also seen earlier in the movie when O'Hara is killed that indicates Lee is "unleashing the beast," so to speak.
"You have offended my family... and you offended the Shaolin Temple." It's an appropriate line for incredible fight scene that followed. Also Lee breaking all the mirrors to get rid of Han's advantage.