My top three are:

Darth Vader’s entry scene: It’s the first time we see him step through the doors of the Tantive IV, and we immediately know that he’s a menacing force. It also gave Vader a great introduction in general: first the stormtroopers, and then Vader himself comes on board to retrieve the stolen plans. Always a great scene to watch.
Princess Leia’s comments about Han and Chewie after they leave the trash compactor on the Death Star: Here you can already see that Leia is not the typical “Princess,” and I love how she’s being a little bratty. She shows that she truly is a royal person, even though she fights for the Rebellion.
The first “Star Wars” fight ever on screen: Ben (Obi-Wan) versus Darth Vader, It’s simply because we saw the lightsabers in action for the very first time, and I really like that the fight isn’t some spectacular acrobatic display, but a personal confrontation for both Vader and Kenobi. This is also an aspect I really appreciate about the original trilogy: they showed lightsaber fights when they were needed, not just when the audience wanted to see one.