“Metropolis” by Lady Frieda Von Harbou.

“Frankenstein” by Mary Shelly (yes, it’s philosophical, but it’s about our relationship to technology, and therefore it’s sci-fi.)

Anything by Jules Verne or H.G. Wells counts.

After WWII, the market started getting really hot. Heinlein, Philip K. Dick, Arthur Clarke, Asimov pretty much controlled the 50′s and 50′s and early 70′s. Most anything by them is either considered Classic or at least a fun read.

Don’t want to leave out Larry Niven, probably the most polite writer a reader could enjoy. “Ring World” is a classic in my view.

Same with “Sirens of Titan” by Vonnegut and “Venus on a Half-Shell” by “Kilgore Trought” (aka Philip Jose Farmer.)

The genre is huge. I’ll let somebody else give you the Fantasy list.

The City and the Stars and the Sands of Mars by Arthur C. Clarke
Childhood’s End by Arthur C. Clarke
The Moon Is a Harsh Mistress by Robert A. Heinlein
Dune (Dune Chronicles, Book 1) by Frank Herbert
Foundation (Foundation Novels (Paperback)) by Isaac Asimov
Stranger in a Strange Land by Robert A. Heinlein
The Stars My Destination by Alfred Bester
Lord of Light by Roger Zelazny
Ringworld by Larry Niven
Neuromancer (Ace Science Fiction) by William Gibson

Do you mean old, like HG Wells
Or “widely read” like Heinlein and Asimov?