Star Trek (TOS, ENT, TNG), SG1, SGU, Firefly, BSG (old and new), Farscape, and Buck Rodgers are my favorite Sci-fi shows. I'm sure I missed a couple.
I am sure being my age, almost 70, has something to do with it, but some my earliest memories are of going to the drive-ins with my parents until I was in the 3rd or 4th grade when I was allowed to go to the movie by myself. I recall few weekends that I did not go to a movie and on many week days if my grades were good, I was allowed to go to a late afternoon show after school.
The films that really stand out from the git'go to me from the Sci-Fi genre are:
1951's Howard Hawks'
The Thing from Another World 
1951's
The Day the Earth Stood Still 
and
1954's
THEM 
While I vivid recall seeing
THEM at the Ideal Theater in Corsicana, Texas, with my cousins Wick and Skip, I am sure my memory of seeing
The Day the Earth Stood Still and
The Thing from Another World were in a re-release during the mid-1950s.
Yet over the 60 plus years since first seeing these, I have never seen another Sci-Fi film that equals either of these three for plot and overall acting excellence. Yes, Special Effects/CGI have come light years from the state of the arts effects used in 1951 and 1954, but while today's films can show literally just about anything one can imagine, they are story poor. Some don't have any plot to speak of that makes sense.
Or like they did when rebooting
The Day the Earth Stood Still, they went full PC and in effect made a rather large pile of steaming you know what that was an insult to the original.
As to other Sci-Fi films from the 1950s, these four stand out to me:
1950's Rebert A. Heinlein's
Destination Moon 
1954's
Creature from the Black Lagoon 
1956's
Forbidden Planet 
1956's
Invasion of the Body Snatchers 
These seven films are both appealing to children of the 1950s and made sense to the adults that also enjoyed them. You don't find that today as what passes for Sci-Fi today is made for 8 to 16 year old boys who will go to a movie ten or fifteen times.
Of course, there were many other mid-to-late 1950s Sci-Fi flicks that ranged from passable even to an 8 to 13 year old to down right silly--
All in all, those at the top of my comments still stand as great Science Fiction films that stand with or above the films of today and tomorrow.