His best-known connection to SF was for his role as Commander JJ Adams in the classic 1956 film Forbidden Planet. I think the first time I saw the movie was as a teen in the late 80's watching the late night Sci Fi Friday feature on KSTW out of Seattle. What struck me immediately was that it was a rare example of smart science fiction storytelling in 1950's cinema (although it had an advantage over most movies, working from the Bard's excellent source material), and it's one that I still enjoy watching (in fact I'm still kicking myself for not picking up a copy on DVD a couple of years ago when they released the anniversary edition). For his part, Nielsen did a good job of playing the typical square-jawed ship captain forced to deal with a deeply weird and extremely dangerous situation.
Nielsen had other SF gigs as well, including the early 50's TV series Tales of Tomorrow (never seen this one because I've never come across any reruns of it and haven't bothered to try to hunt it down online).
Mainstream audiences probably remember him best for his comedic roles of the past 30 years, including Airplane! and the Police Squad TV series and its successor Naked Gun movies. The Naked Gun flicks were pretty funny, but I also really enjoyed Nielsen as the 'shroom-fuelled Jedi master of curling in Paul Gross' Men with Brooms. Come to think of it, the only one of Nielsen's comedies that didn't make me laugh (surprisingly) was the Mel Brooks flop Dracula: Dead and Loving It. Overall though, Nielsen had a brilliant sense of timing, the ability to keep a straight face amidst the most ridiculous antics, and a twinkle in his eye that always promised more laughs.
His accolades include being made an Officer of the Order of Canada in 2002, and stars on the Hollywood and the Canadian Walk of Fame.
Leslie Nielsen was 84.