I was just rewatching the original Thin Man movie and it's still a timeless classic.
Out of all her co-stars I'd have to say it was Willaim Powell. The two of them had some incredible chemistry, put them together and it was magic on the screen. Second only to William would be Clark Gable. The two of them were great together.
And if I had to choose a third I'd probably say Cary Grant but part of that has to do with I've always liked him as a leading man and can honestly say that I've never disliked him in any movie he was in.

William Powell
Clark Gable
Cary Grant
Melvyn Douglas
Spencer Tracy
In that order although while William Powell wins almost all the way, there is something about the Gable/Loy pairing too. She had great chemistry with both men, even when she and Gable weren't speaking to each other.

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I think her best co-stars were William Powell, Cary Grant and Clark Gable.
I wish Myrna had made more movies with Leslie Howard, Tyrone Power, Henry Fonda, James Stewart and Fredric March.

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William Powell, of course!!!
"You can't HANDLE the truth!" Jack Nicholson, "A Few Good Men."

Without a doubt William Powell. I do like her with Cary Grant, as well. When TCM showed 3 of her movies in a row, 2 were with other actors and the other was Evelyn Prentice, EP was definitely the more memorable one because of their chemistry. While I enjoyed her performances in all 3, the other two blended together.

William Powell
Its that man again!!

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She overacted when not with Powell? Are you crazy? She has to be one of the most laid back actresses ever.

Even though I haven't seen as full a range of Myrna's movies as I'd like (her MGM movies need to be released pronto!), I'd have to say that her best male co-star is William Powell. As much as I love her, she had a tendency of overact, and when she was in movies without him, her mannerisms were sloppy and the dramatic/emotional scenes bordered on hysteria. Somehow, Bill seemed to smooth her rought (acting) edges and she came across more naturally, and her humor didn't seem forced.
http://edwardianpromenade.com

William Powell
Cary Grant
Clark Gable
Spencer Tracy
Melvyn Douglas

Has to be William Powell, those two were the best double act in the history of cinema.

No doubt William Powell.

William Powell, of course. The best screen pairing of all time.
Faith can move mountains, but dynamite works better.

What kind of question is that? William Powell! They've got to be my favorite comedic screen couple.
Paris is for lovers. Maybe that's why I stayed only 35 minutes.

Gable by far
Viva Clark Gable, el eterno y nico Rey de Hollywood

Definitely William Powell.
I've seen the 6 Thin Man films, Love Crazy and Libeled Lady so far and will be getting all 14 of their films together on dvd eventually.
followed by Cary Grant (Bobby Soxer, Blandings) and then Clark Gable (Test Pilot).

If you're watching 'Fullscreen' DVDs, you aren't getting the whole picture.

U R right, MerryMyrna. TV has tried, with HART TO HART and McMILLAN & WIFE, but none of them could hold a candle to Bill and Myrna (and Asta).

In my mind it can't be anyone other than William Powell!! I love those two in The Thin Man series.Wonderful, playful chemistry that few matched.

William Powell, with absolutely no after-thoughts! She had great chemistry with many actors including Clark Gable, Cary Grant and Robert Taylor, but the Powell & Loy pairup is something we'll never be able to see again. Ever.
"Life, is not a having and a getting, but a being and a becoming." - Myrna Loy

Frederic March in
The Best Years of Our Lives
, because you really believed they'd been married for twenty years.
"Well, for once the rich white man is in control!"
C. M. Burns

my favourite male myrna loy costar is william powell followed by asta the dog!
cheers.