Just wondering, who's your favorite actor  

Zasu Pitts?
No...maybe....
Katharine Hepburn
Carole Lombard
Cary Grant

Cary Grant as Diamond thief
in 1955's "To Catch a Thief" in the role of John Robbie the "Cat" also starred that Philadelphia "adventuress" Grace Kelly. Great South of France locales , one of my all time faves.

Thanks BigTelevision, that's ...
Thanks BigTelevision, that's the one. I'm a dope for dogging the acting but...Grant, et al, deliver their lines in a way I have a hard time believing actual people speak or spoke. The locales are great, though, the outdoor scenes especially - the one I recall in the boat and the one in the car with GK racing along the road were, if I recall correctly, pretty obviously studio wrapped, which unfortunately adds a little something to that unbelievability factor.

Oh dear.
This is such a tough question.
I think most of the obvious have been namechecked but I'd like to give a special mention to Peter Lorre. Gloriously underrated and always overlooked.
....and while i'm here i'd like to thank Burt Lancaster for over-acting throughout his entire career and compliment him for those wonderfull minutes of suspense during the tunnel sequence in 'The train'.
Another favourite is the partnership of Marie Windsor and Elisha Cook Jr in Kubrick's "The Killing'.
More up to date I feel that we have not yet seen the best from Philip Seymour Hoffman ( fantastic in ' Love Lisa' ). Thanks and praise to the Coen brothers off course and hats off to Frances Macdormund's hilarious performance in Fargo.
I could go on all day.....

What was even more creepy
in "To Catch a Thief" is that Grace Kelly is racing along the Corniche in the South of France and thirty years or so later she would die on those very same roads.

Wow, I recall she died in a...
Wow, I recall she died in a car accident, but I don't think I had any idea it was in the location of that film. I don't know whether she was "acting", but it didn't look like in the movie she had the mechanics of driving down very well. The terrain looked absolutely treacherous, too. I think I recall hearing or reading, in an interview or book, that Cary Grant said Grace Kelly was his favorite actress.

.
"The Train" -- a magnificent film -- takes me somehow to "Ship of Fools": both have iconic French actresses and other worthy actors, shot in time-honored and atmospheric black and white. But only one of those films has Vivien Leigh, Simone Signoret, José Ferrer, Lee Marvin, Oskar Werner, Michael Dunn, Elizabeth Ashley, George Segal, José Greco and Heinz Rühmann. Take your pick. . .