Sci-fi movies are my forte, so I've got a long list. I'll share a few of the lost and often-overlooked gems (IMO). Some of these may or may not be on Sci-Fi fans' radar, but I'm looking mostly from more of a mainstream perspective.
Vicious Lips (Campy 80's film about an all female band in space)
Lost In Space (Largely forgotten and panned big budget 90's movie but I enjoy it)
Reign Of Fire (Another largely forgotten big budget movie with Matthew Mcconaughey and Christian Bale fighting dragons)
Snowpiercer (This movie seems to be popular on Netflix, but since it was a foreign production it didn't gain too much traction in the mainstream - it's one of Chris Evans' best performances I thought)
Barbarella (Gotta love 60's Jane Fonda)
Evilution (David Duchovny and Julianne Moore fighting aliens with shampoo = campy fun)
Frequency (son Jim Cavieziel and father Dennis Quaid are linked through a ham radio present and past and try to prevent the father's death)
Frequencies (People are bonded or prevented from bonding through frequencies they emit - very emotionally gripping)
Eva (A foreign film similar to Ex Machina, but perhaps more emotionally gripping as the main character's daughter is a robot)
Virus (Another overlooked 90's bigger budget movie - Jamie Curtis and her crew run into a Russian ship overrun with a virus that creates deadly machines)
Event Horizon (Seems to be pretty popular on Netflix, but overlooked elsewhere - I thought it was very scary the first time through)
Pandorum (I think this was overlooked as well when it was new, but it really delves into the potential psychological tolls of space travel and has an awesome twist)
Heavy Metal (Excellent 80's animation film based on the magazine - with an awesome soundtrack)
The 6th Day (Schwarzenegger gets cloned - "He's smoking my stogies!")
Ragnarok (Foreign film about an archeologist that unearths a giant monster)
Terminal Invasion (Bruce Campbell at his campy best - protecting an airport terminal from aliens that take human form)
I could go on, but that's already a lot.