To me, Greenstreet 
was one of those few actors that had brief but stellar careers in films that gave their characters gravitas, making them stand out far and above the established super stars with which they shared the screen--
Ranking a hair above Sidney is Clifton Webb 
who was the ultimate urbane high society snob who sometimes, reluctantly offered up a heart of gold. Laura without his Waldo Lydecker would not be remembered as the film classic it is, nor would any of his essential characters in The Razor's Edge, Sitting Pretty, Cheaper by the Dozen, the three 'Mr. Belvedere' films, Cheaper by the Dozen, Dreamboat, Stars and Stripes Forever, Titanic (to me a much better story than James Cameron's special effects extravaganza), Mr. Scoutmaster, Woman's World, The Man Who Never Was and The Remarkable Mr. Pennypacker.
Besides these two there was Laird Cregar 
who was brilliant in way too few films he made from 1940 until his sudden death in 1944 at age 31.
Yes, there was a different style of film acting in those sixty, seventy some odd years ago, yet in many ways it was better than the stark realism offered up by today A-List super stars. And, of course, the plots and story lines were much, much better that what is offered up today. Few of the films made today even have a plot, or even one that makes sense--
Where or where is that damn time machine so I can go back to better days?
Gadzooks-- Was... 
I lucid dreaming again?