As always, I'll preface this by saying that I'm hazarding an opinion on this matter with very insufficient evidence.  But I'll be intrepid here and say that I like the 50s quite a good deal.

First and perhaps most importantly, this is sort of the end of the great studio period in Hollywood.  You've got established masters in their prime:  Hitchcock (Vertigo, Rear Window, North by Northwest, etc.); Hawks (Rio Bravo, Gentlemen Prefer Blondes); Lang (The Big Heat, Beyond a Reasonable Doubt, etc.); Ford (The Searchers).  You've got the end of the noir cycle with Kiss Me Deadly and Touch of Evil.  You've got the start of the careers of Ray, Fuller, Kubrick, etc.  And you've got your occasional brilliant Hollywood one-off like The Night of the Hunter.  

Second, you've got major things happening in France.  Most importantly, you've got the first New Wave films (e.g. Breathless, The 400 Blows, etc.) at the end of the decade.  You also have other masters--like Renoir (The Golden Coach, The River, etc.) and Bresson (A Man Escaped, Pickpocket, Diary of a Country Priest--doing their thing.

In Japan, you've got the late and very rich work of their two greatest directors (at least in my opinion), Mizoguchi and Ozu.  Also, you've got what might be Kurosawa's richest period.

In Italy, you have late neorealism.  You also have Rossellini's work in this period, as well as the next generation of Italian masters (e.g. Antonioni, Visconti, and Fellini) coming into their own.

Plus, you've got major work by directors elsewhere.  Dreyer's Ordet and Bunuel's Los Olvidados come to mind at the moment. 

I'm sure I'm forgetting a lot of stuff I should be mentioning, but that seems like a pretty good decade to me as I look at it now.  (By the way, I think respectable cases could also be made for the 20s and the 60s.)
