Obviously the Star Wars Ewok Adventures are the best.
One of those was the first Star Wars movie I saw as a kid. So I thought Star Wars sucked and had no interest in seeing any other. It wasn't until years later that a friend got me to watch A New Hope that I realized Star Wars was actually good.
Anyways I like the original cuts of A New Hope and Empire Strikes Back the best. Before Lucas took a steaming dump on them with CGI and basically gave fans the finger about any HD version of the originals. The Return of the Jedi is a bit drawn out. I don't particularly care for the prequels. The young Vader's archetype and sudden shift makes no sense. The new one is pretty good I've only seen it once so I don't have a strong opinion.
As for favorite Sci Fi movie. That is a much tougher choice. Their is a broad scope of categories.
For an Earth based dystopian future. I'd go with "Soylent Green". As the masses have really just become useless dead weight and a burden to be dealt with. "Logan's Run" is fun but the 30 year age cap sounds less plausible.
For calamities caused by science. I'd have to go with "Planet of the Apes" (original). Although "The Omega Man" is very close.
In the realm of Time Travel. I'd go with the original two "Terminator" movies. Although "Back to the Future" is a very close second place.
As for alien invasion movies. I'll stick with the original, "The Day the Earth Stood Still". Although Spielberg's "War of the World's" is quite good. "The Day the Earth Stood Still" is a bit more plausible as the threat is ended by diplomacy. Any movie where Earth wins the fight are implausible but fun to watch. All the makes "War of the World's" somewhat believable is that disease not man won the day.
In the general realm of space base Sci Fi. I like "Star Trek: The Motion Picture" best. I know it is way down the list for most Star Trek fans. They deal with a completely unknown entity which is a major threat. The threat is nullified by understanding and ultimately sacrifice not battle, cunning or tech miracles. I do like the other original crew movies as well. The TNG movies aren't watchable and the new ones while entertaining have no feeling or character. The original "Alien" is great but leans more towards horror than Sci Fi.
Although they don't make this list. I do favor John Carpenter's "The Thing", "Forbidden Planet", "The Andromeda Stain" and "Invasion of the Body Snatchers" (1978).
Star Wars while entertaining. Has very little science. It is more of a tragedy or fantasy set in space. It focuses much more on mystical powers. Little explanation is given to the technical aspect. There are simply very fast space ships, energy swords and powerful weapons.
