I need some time to think up a good, concise list...BUT IN THE MEANTIME...

Leo Dicaprio's performance in "What's Eating Gilbert Grape" is one of the best acting performances that I have ever seen.

In fact, (since this was before he was an A-lister) I remember coming out of the theater and wondering if they had trained a mentally-challenged actor for the role.

Two more I just remembered:

Jack Lemmon & Walther Matthau - The Odd Couple

NinaB said:

Lawrence Fishburne & Jeff Goldbloom in Deep Cover. Christopher Walken in King of New York & Deer Hunter. Al Pacino in Frankie & Johnny & Sea of Love. Jack Nicholson in Witches of Eastwick & Batman. Billy Dee Williams in Lady Sings The Blues. Richard Pryor & Gene Wilder in Stir Crazy. Jodie Foster in Bugsy Malone & Silence of the Lambs. Julie Walters & Michael Caine in Educating Rita. .....& loads more..

I forgot about that movie but I'll have to say Diana got robbed of the Oscar. She was amazing in that film.

UncleJam said:

Many of my faves are listed, so...

Val Kilmer as Jim Morrison in "The Doors" and Doc Holliday in "Tombstone".

Jackie Gleason as Sherriff Buford T Justice of Portague County, TX in "Smokey and the Bandit" (only the original; the sequels...not so much)

Matt McConnaughey (sp) as Wooderson in "Dazed and Confused".

Mr T as Clubber Lang in "Rocky III"

[Edited 2/4/14 19:35pm]

OMG forgot that one too. Val WAS Jim!!!! I forgot I was watching an actor. Almost like Jamie Foxx and his depiction of Ray Charles!

will smith men in black/wild wild west/ independance day

larence fishburne/christopher walken KING of NEW YORK

LF MATRIX, BOYS IN THE HOOD, HOODLUM

Jack Nicholson's scene in Five Easy Pieces, which is, IMO, the best God and Man allegory ever filmed.