Some are profound philosophical statements, some are artistic masterpieces, some are great entertainments, some just influenced me as a youth. All are worth reading again. (Again, not in any particular order)

The Odyssey - Homer (I recommend the WHD Rouse translation, still available in paperback)

Lord of the Flies - William Golding

1984 - George Orwell

History of the Peleponesian War - Thucydides

The Old Testament - (KJV, for the quality of the prose)

Portnoy's Complaint - Phillip Roth

The Origin of Consciousness in the Breakdown of the Bicameral Mind - Julian Jaynes

The Magus - John Fowles

Starship Troopers - Robert A. Heinlein

Moby Dick - Herman Melville

Beat to Quarters - Cecil Scott Forester

Flashman - George Macdonald Fraser

The Challenge of the Spaceship - Arthur C. Clarke