I went with Marion and Indy. I think the chemistry between them makes that scene resonate a little more for me, because you can really feel the passion between them. It's set up so well too, because we know they were together years before and their feelings for each other are developed right from the start when Indy mentions her while packing to go to Nepal and continues being fleshed out through the middle of the movie. Then the scene on the ship seems like it's going to be the payoff, and Indy falls asleep. So classic.
The scene with Willie is really good too, but I agree with the poster who said it seemed contrived. There was no setup for them to all of a sudden want to jump one another - one second he just seemed annoyed by her (even though he thought she was beautiful), and the next second he's putting the moves on her. I don't know, it's a small gripe, and I still like the scene, I just think the one with Indy and Marion is better.