Alien, Blade Runner, Moon, Solaris, 2001 and The Thing are all top films.
I'll watch any old guff but personal faves include..
Repo Man, Silent Running, Event Horizon, Upstream Colour, Brazil, Lifeforce, Source Code, Cold Souls, Cypher, Serenity, Children of Men, Pitch Black, Contact, Ghost in the Shell, Westworld, The Terminator, The Fifth Element, Gattaca, Primer, 12 Monkeys.
can't resist...
Alien 9/10 (prob a top 20 this)
Blade Runner 8/10 (Final Cut version....top 50)
Moon 7/10 (reminds me of an Asimov story)
Solaris 7/10 (if you mean the Tarkovsky original...4/10 for Clooney's effort)
2001 10/10 (top 1)
The Thing 6/10 (due a rewatch, only seen it once and that were about 20 years ago)
Repo Man 6/10 (ditto)
Silent Running (not seen but have it on DVD so soooon)
Event Horizon 7/10 (great atmosphere)
Upstream Colour (never heard of this...on the watchlist it goes!)
Brazil 5/10 (gave me a headache)
Lifeforce (another new one for me...sounds fun!)
Source Code (not seen it but another one already on the DVD shelf)
Cold Souls (not heard of this either, decent tip that)
Cypher 3/10 (started off well...shame)
Serenity (got it on DVD, haven't seen yet as thought maybe I should go through Firefly first)
Children of Men 9/10 (top 20)
Pitch Black 5/10 (bang-average...is Riddick any better?)
Contact 6/10 (suffers from the same problem all 2001-pretenders do...too much emotion/focus on relationships)
Ghost in the Shell 5/10 (too hectic)
Westworld 7/10 (70's sci-fi is so beautifully dystopic)
The Terminator 8/10 (still a thrill)
The Fifth Element 7/10 (silly, daft and fun)
Gattaca (not seen yet, but have it so also sooon)
Primer 7/10 (clever take on time travel that)
12 Monkeys 7/10 (bonkers but darker and more interesting than Brazil)
Some of my sci-fi favourites which maybe you've not seen, all worth at least a 7/10: Forbidden Planet, Village of the Damned (1960 version), Quatermass and the Pit (1967), The Andromeda Strain (1971), Clockwork Orange, Soylent Green, Close Encounters of the Third Kind, Invasion of the Bodysnatchers (1978), The Black Hole, Altered States, 1984 (John Hurt version), D.A.R.Y.L, The Quiet Earth, Enemy Mine, Transformers (1986), Akira, The Abyss, Fire in the Sky, The Arrival, Cube (ignore the sequels), Dark City, X-Files (1998 movie), Save the Green Planet, The Man from Earth, District 9, Pandorum, The Fourth Kind, Errors of the Human Body, Edge of Tomorrow