Q from Star Trek
7 of 9 from Star Trek
Captain Picard from Star Trek
Fry from Futurama
Ripley from Alien
Master Chief from Halo
The Flood from Halo
Terminator from Terminator
Optimus Prime from Transformers
Dr Strangelove from Dr Stangelove
Dr Peter Venkman from Ghostbusters
Carol from The Walking Dead
When I was a teenager I was quite fond of Ashe, Fox, Murdock and the bunch of explorers from Andre Norton's Time Traders series because they were brave, forthright, smart and adaptive. I was always some disappointed she didn't continue the plot line and fell deeper into "Sword and Sorcery". Once I hit college I found heroes far more complex, adult and real. The most memorable for me are by two authors, Poul Anderson and Larry Niven.
Poul Anderson created two very similar characters in Dominic Flandry and Nicolas Van Rijn. They are not entirely likeable and most certainly not 2-dimensional cardboard cutout heroes but they are completely unapologetic, a bit ruthless and manipulative but basically good-hearted, highly intelligent and fiercely alive.
Somewhat similar is Larry Niven's Louis Wu who due to the longevity possible from "the spice" has realized that Mother Nature is done with him since he is past reproducing offspring and he concludes "out to kill him". He, too, fights fiercely to both stay and be alive (after a decade or so of escapism on "the wire") and "will not go gentle into that sweet night". Wu benefits also from being part of such a vast concept and "what if?" view and storyline world that is Ringworld but is such a fully fleshed-out person he would stand out in almost any story.
Video characters leave less room for "reading in" and so for me are a little harder to maintain a clear image over many years of who they are and/or represent but Picard and Data, for very different reasons, do come close. Of course, Seven of Nine and T'Pol are extremely memorable but mainly because they are just so damned HOT! :P
In no particular order:  Tom Baker  Dr Spock  The Mule (in Foundation & Empire)  Robbie the Robot  Pink Eyes (in Dragon's Egg)  If there were a characterful onboard ship's cat called Schrodinger, that'll be on the shortlist for my favourite SciFi quadruped.
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