You never know. I read one time Marliyn Chambers was making a soft core (no penetration) porno movie about fucking a DJ at a radio station. No matter how they positioned the camera you could see the guys cock hanging out. She said what the hell and just put it in so you couldn't see it. A real trooper there. We miss her.
Girl-girl POV scene in Black Swan, naturally. I saw it at the theatre and wasn't expecting a 40 foot tall pussy licking in HiDef.
I think it's worth mentioning the BJ scene in 'Dragon Tattoo'. The crazy rape scene and ultra violent rape revenge scene get lots of attention, but the BJ scene is the first one and it's a really really slow paced and quiet scene that shocks you because it doesn't go where a Hollywood movie normally goes.
Another sex scene which is 'bad' sex, but good movie making is the Chlöe Sevegny scene at the end of the movie 'Kids'.
She pops up again in "Boys Don't Cry".
If you're looking for an obligatory sex scene where the leading man and leading lady get it on just because they're supposed to, then these aren't what you mean by 'best'. But if you're looking for scenes that shock the viewer and are important to the plot, these should be included on the list.
Mickey Rourke and Denise Huxtable (I mean Lisa Bonet) in Angel Heart, a seriously underrated flick. Bill Cosby was pissed when the film came out because she played his teenage daughter on TV and she did a nude sex scene. Beautiful girl....
Also, if I remember correctly, in that Kathy Shower film (Tennessee Buck) soon after the clip from earlier she gets taken from behind by the chief of the tribe while tied up. The first time seeing that movie was a little shocking too, lol. It was a goofy, campy movie, and the sex scenes were unexpected.
The scene mentioned from Bound with Gershon and Tilly is another memorable one. Jennifer Tilly is fucking sexy!
There was a film that was a blatant ripoff of "Fatal Attraction" called "Body Chemistry" that had some pretty realistic looking sex scenes. The female lead was not anyone that particularly stood out, and the guy was Marc Singer who is most known for playing the title role in the movie that has been probably shown on TBS more times than any other: "The Beastmaster"
I am reminded of a sex scene but it is more like a rape scene. In "the Island" Michael Caine is taken prisoner by some modern day pirates. They are steril so to reproduce they take men for sperm donors. He is in a hut when an attractive pirate woman comes inside. He is bound on the floor and she removes his pants. She takes a large hand full of some kind of grease and starts to massage little Michael. He tried not to respond but you know... She pulls her raggity skirt to one side and she mounts him. It doesn't take long for her gyrations to get him off.