Two that comes to mind.

Silver Streak-1976 Gene Wilder and Jill Clayburgh. It was dramatic for an hour until Richard Pryor showed up. Then it was full of action. I wouldn't say Richard kind of carried the movie, but he added that twist that made the movie special.

Ghost-1990 Patrick Swayze and Demi Moore. This movie would have never as been good had Whoopi Goldberg been great in the supporting role. She didn't win best supporting actress for nothing that year.

Junebug - Amy Adams

They Drive by Night (1940) in the film, a truck driver (George Raft) is coerced into a murder plot by a beautiful seductress (Ida Lupino); but, it's Humphrey Bogart as the trucker's younger brother who really shines in this film; soon, he would be a bigger star than either Raft or Lupino.

Does Aaron Eckhardt in the Dark Knight count?

Pulp Fiction I would say Samuel L. Jackson stole the show from both John Travolta and Bruce Willis.

Bringing Down the House-Steve Martin had the starring credits of the movie, but Queen Latifah was the real star. The movie would have been boring if not for her and would have collasped.

Junebug - Amy Adams

I saw that movie sometime back I agree Amy's character carried the movie. I hated the way some of the other characters treated her.

"Aladdin" (with Robin Williams as the genie)
"Alice in Wonderland" (with Ed Wynn as the Mad Hatter and Jerry Colonna as
the March Hare)
"My Friend Irma" (with Dean Martin & Jerry Lewis in their first film)
"Babes in Toyland" (with Ed Wynn, Henry Calvin and Gene Sheldon)
"Li'l Abner" (with Stubby Kaye as Marryin' Sam)
"The Little Mermaid" (with Buddy Hackett as Scuttle)
"Splash" (with John Candy as Tom Hanks' brother)
"Star Trek II: The Wrath of Khan" (with Ricardo Montalban as Khan)
"Batman" (1989; with scenery-chewing Jack Nicholson as The Joker)
"Shrek" (with Eddie Murphy as Donkey)

Sixteen CandlesThe Geek, Anthony Michael Hall stole every scene he was in. The character Long Duc Dong from the same movie also stole the scenes he was in, as well as the kid who played Molly Ringwald's little brother Mike.

The Breakfast Club Judd Nelson as the degenerate stole all the scenes he was in.