She was given a second Oscar for her performance in "Guess Who's Coming to Dinner," probably because many in the Academy expected it would be her swan song. But she fooled them, and they had to give her yet another Oscar the following year for "The Lion in Winter," not out of sentiment, but because she deserved it.
Granted, I haven't seen all of her movies and so I'm biased when I say this, but her performance in The Lion in Winter was my favorite of hers and of many movies. She was so amazing in that movie. And it's a thrilling movie too.