Here are some of my favorites:
Starman -Jeff Bridges (at his hottest) as an alien who takes the form of a woman's recently-deceased husband.
The Andromeda Strain -A team of scientists at a secret facility scramble to find a cure for an alien organism that has killed an entire town.
The Day the Earth Stood Still (original, not the remake) -An alien lands in Washington DC, threatening to destroy the Earth if we bring a atomic weapons into space. An absolute classic.
Alien Nation -In the near future, the aliens of landed and now they are being integrated into society. This one centers on the first one to become a police detective. James Caan and Mandy Patinkin star.
The Final Countdown -The aircraft carrier USS Nimitz goes through a time storm at sea, and finds itself off the coast of Hawaii on the eve of the attack on Pearl Harbor.
The Omega Man -Charleton Heston as one of the last survivors of a plague that has killed off most people. The remaining have become ghouls. An adaptation of Richard Matheson's I Am Legend.
Planet of Storms -A classic of Russian cinema. It's been chopped up and dubbed as a Hollywood B-movie several times, but the original is actually a wonderful, intelligent sci-fi film. Available on DVD, in Russian with English subtitles.
Robinson Crusoe on Mars -The classic story, updated to a manned Mars mission. Friday is an alien escaped slave. Paul Mantee shows off his fantastic, furry chest and hot bod in many scenes. Adam West (Batman) has a small role.
Them - Another classic. Giant, mutant ants invade LA. Blink and you'll miss a very young Leonard Nemoy in a one-line role. Maybe the best of the 50s atom bomb films.
The Time Travelers -A 60s B-movie favorite about a group of scientist who create a portal to the far-distant future, but then become trapped there. You probably saw it on Saturday afternoon TV as a kid. Finally available on Blu-ray.
Where Have All the People Gone? -A made-for-TV movie that is available on DVD and on YouTube. A family is exploring a cave when mysterious solar flares strike. They emerge to find all the people gone -just empty clothes filled with a white powder. A quest to return home and start a new life.