I guess the place to start for SF comics, particularly on a British site, is 2000AD. Its title now makes it sound very unlike SF, but it’s been running future adventure stories for decades. It’s never been consistently great, but it’s had lots of great strips over the years: Alan Moore and Ian Gibson’s future-Locas series Halo Jones, Grant Morrison and Steve Yeowell’s superhero strip Zenith, Pat Mills’ future-inquisition story Nemesis, with lots of artists, but most famously, Judge Dredd. I don’t know how many Dredd stories there have been by now, but nearly all of them are at least pretty good – Mills and John Wagner managed a strong standard for a very long time. It’s hard to know where to start with highlights, but the early Judge Death stories, with art by Brian Bolland, are wonderful (a sample is shown, a favourite comic moment of mine), and Mike McMahon’s art in the same era is as good as British action art has ever been – well, except he may have beaten it on Pat Mills’ Celtic fantasy series Slaine, also in 2000AD.
SF’s always had a big part in comics all over the world – the argument used to be that comics had an unlimited special effects budget. Whatever, there have been some good ones. EC did plenty of SF, with some wonderful art – see the horror and war entries for some of the names. The stories are often a little unexciting, but they are very nice to look at. Before Marvel started doing superheroes, the great Jack Kirby and Steve Ditko did lots of SF and horror for them. Kirby specialised in big monsters, Ditko in little twisty tales. These are sillier and more fun than the EC stories, and the art is even better – Ditko in particular provided a lot of the best splash pages I’ve ever seen on these stories.
It’s also been a huge part of manga. My taste for big robots and post-apocalyptic settings is pretty small, so most of it is of no great interest to me, but I do want to mention Hiyao Miyazaki’s Nausicaa of the Valley of the Wind – he wrote and drew the comic, as well as directing the movie. It’s a dense work, much more complex and substantial than the film, and as strong an SF story as I’ve ever read in comics.
Collections: all of Nemesis is collected in fat, reasonably priced volumes, and the same publisher is gradually putting out Judge Dredd: The Complete Case Files (10 books so far – these are nearly as good value, pages per pound, as Marvel’s Essential and DC’s Showcase reprints). Moore’s Halo Jones and the McMahon Slaines can be found reasonably easily in book collections too. The EC SF has been reprinted more than once, and there are big hardback volumes too. There’s a lovely big omnibus collection of Amazing Fantasy, including some terrific Kirby monsters and a huge amount of Ditko SF and horror shorts. Nausicaa is available in four books.