The one's where the accent sounds natural.
Peter Finch Network, he's apparenlty very british but you couldn't tell me that after watching network.
Jullian Mcmahon Fantastic Four (morely nip/tuck, but its the same voice) He has an austraillian accent.

All Australian male actors who play Americans.

Bob Haskin's American accent in pretty much anything.

I remember reading that Renee Zellweger's accent from the Bridget Jones movie was very widely accepted as being extremely natural sounding.

Claude LaMont from Kentucky Fried Movie.

I am zee unknown...

I don't think so. It sounded very put on to me, and i'm english. I think a better try was by Angelina Jolie in the Tomb Raider films or even Natalie Portman in V for Vendetta. I personally think that Martin Landau did an amazing job altogether in Ed Wood. Deserving of the Oscar.

Brad Pitt, Snatch

Christian Bale in American Psycho (and I guess any other movie where he plays an american). I remember being stunned when I watched the extras on DVD and heard an on set interview using his real accent.

Toni Collette in everything.

How about people who've lost their accents :

Mel Gibson
Anna Paquin

Al Pacino in Scarface. Ok, so I've never actually heard a Cuban talk that way, it was still pretty unique.

Gael Garcia Bernal in Bad Education

Steven Culp as Robert Kennedy in Thirteen Days. His Kennedy accent was just perfect.

What kind of accent did she formerly have?

For me it's a tie: Laurence Fishburne in The Matrix and... Laurence Fishburne in Higher Learning.

Well even though Mel Gibson lived in Austrailia for some time he was actually born in America. So i guess he lost his accent very early on, picked up a new one, and then went back to his original accent.

As for Anna Paquin...well after appearing in the Piano with a strong Aussie/New Zealand accent it is not surprising that she lost it. I can imagine after she won the oscar for her role she immediately moved to the US and has lived/worked there ever since.

I don't get you stating Laurence Fishburne in the Matrix??? Its his normal voice that he speaks with. Not seen Higher Learning so couldn't coment.

That was the first thing that came to mind.

Christopher Guest / Harry Shearer / Michael McKean - This Is Spinal Tap

I was half joking because I actually got a laugh from both accents. In the Matrix he has a very slow, deliberate delivery that sounds lower than his regular voice.

Although not a movie, Hugh Laurie's American accent on House M.D. is perfect. I think it's funny that even Bryan Singer didn't even know that he was British until he was told by one of the other producers.

That is the singlehandedly worst Cuban accent EVER committed to film.

He had a Jamaican accent in Higher Learning.

Claire Forlani in just about anything. I never knew she was a Brit til years after I saw Mallrats.

Anthony LaPaglia is another actor who falls into that same category. Although his Aussie accent has all but disappeared at this point when you hear him in interviews.

"The dingo stole my baby!"