50's era Heinlein is a good intro to newcomers.
Any of the juvenies if they're young; THE MOON IS A HARSH MISTRESS if they're older.
His first novel (BEYOND THIS HORIZON) is probably a bit better for sheer accessibility.
Asimov? NIGHTFALL is probably a good intro to what he's all about.
Clarke? Oh, go straight to CHILDHOOD'S END.
And since Herbert keeps coming up; yes, do DUNE. Avoid all sequels, avoid SANTAROGA BARRIER, HELLSTROM'S HIVE and most of his work other than UNDER PRESSURE, at least until you're familiar with the genre. Accessible he ain't.
Bradbury is in many ways the opposite of Asimov and Clark; more about mood and style than idea and concept. A SOUND OF THUNDER is probably as accessible he gets.