Frank Herbert's "Dune" is the greatest scifi story of all time.
I don't get why people recommend dune as sci-fi, although it
technically is, its much more science-fantasy really. I barely made it through the book because I just found the politics boring.
The first and maybe second books are good, but in the 3rd the dude just started writing fanfiction about little girls having sex with everything (including giants) for some reason. Had to drop it after that.
I recommend Arthur C. Clarke's stuff, he ignores the 'boring' stuff and writes sci-fi that focuses on the larger story instead of characters, so if that's an issue just warning. I really enjoyed 'Rendezvous with Rama', it really feels like an adventure. Can't recommend the sequels that came after though, they ruined the mystery and just feel like a slog to read compared to the first.
Another series that skirts the line as being sci-fi, but is similar to Roadside Picnic is the Southern Reach trilogy by Jeff Vandermeer (hes a woke cuck on twitter, just warning, doesn't come across in his work though). They are less sci-fi, more similar to weird fiction in the vein of Lovecraft, specifically 'The Color out of Space'. It's basically a story about exploring the unknown, with a very good interpretation of an alien, which is so different from us that we can't comprehend it.
'Gateway' by Frederik Pohl is something sort of in the vein of Roadside Picnic, in that its about exploring the remnants left behind of aliens, but used as a tool to explore the galaxy. Has more characterization and presents some interesting concepts.