During the 40s she had 4 great roles, but only two oscar nominations. Those happened for The Philadelphia Story and Woman of the Year, while State of the Union and Adam's Rib (eligible 1950) was also deserving for some award especially the underrated State of the Union. But well Katharines 1940s is like Meryls 1990s, they did good roles but nothing comparable to their roles in the decade before or after.
So to the much happier 50s. This decade Hepburn landed some of her best roles and movies. The African Queen, Summertime and Suddenly Last Summer all three stands as great movies, but only Summertime is a real masterpiece. But on the other hand the Hepburn manner became more and more prominent, which you could notice very well in The African Queen and Suddenly Last Summer. Summertime on the other hand Hepburn gave a fine emotional and natural performance as the old spinster who finds love in Venice.
The 60s she only did 4 movies roles, but they were chosen carefully. Long Day's Journey Into Night IS an essential from stage to screen performance and Hepburn is just brilliant. Guess Who's coming to dinner isn't maybe anything groundbreaking but still a fine and relaxing performance. In The Lion in Winter she heated the performance up to such a degree that it sometimes becomes quite Over-the-top, but you never get annoyed of this.
In the 70s-80s her career seemed to decline with her age and the best performance from the decades was The Trojan Woman, an exciting performance from an all-time great actress.