Your favorite...

Most of what's been mentioned so far I would classify as Sci-Fi rather than horror.

The big Sci-Fi movies of the 1950s are favorites of mine. I couldn't pick just one. I've seen them so many times!

I guess "Them!" and 1956 "Body Snatchers" and the 1952 "The Thing" and 1953 "War of the Worlds" and the "Black Lagoon" films would be at the top of my list. But, there are still lots more that I can't resist watching whenever I come across them.

Another good one from a later decade is 1988 "They Live"

If you are also a fan of "They Live," I recommend reading the original short story that it is based on: "Eight O'Clock in the Morning" by Ray Nelson.
 
As a Chicagoland native, been tuning into Svengoolie since the late 70's. Haven't stopped watching, well except when it was off the air. (That era is known as my second Three Stooges period.)
Anyway, the Hammer Studios & Universal classics. Vincent Price in House of Wax. Killer Klowns From Outer Space.
1971 Duel
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And The Abbott & Costello monster movies.
Oh, and those 1970's made for TV movies Sven runs. The one about the ants was really good.
Ants !
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