Most unabashed nude scene?

Now And Later, Shortbus and Desire all contain a great deal of nudity and unsimulated sex (although none of the actors are household names)

Jane March in either The Lover (1992) or Color of Night (1994).

Swearnet: The Movie (2014). There is a scene that takes place at a televised hockey game. A man is hanging off a railing and somehow loses his pants. He somehow lands crotch first into the camera that is filming the game. It cuts to the POV of the camera and you see his dick squishing against the camera. You see everything in detail.

Side note: this film is in the Guinness World Records for most uses of the word "fuck" in a film and averages 8.35 times a minute.

I always appreciated Katherine Beller's nude swimming pool scene in The Betsy as Tommy Lee Jones watches from the balcony.

Not necessarily "most in your face" but Alexis Dziena in Broken Flowers(2005) springs to mind.

Sasha Baron Cohen in Bruno and Borat

Michael Fassbender’s dangling about a lot in Shame (2011).

Anne Hataway in Love and Other Drugs