1984 by George Orwell, it's a book that changed our World I believe and coined more than a few words/phrases.
Gulliver's Travels by Jonathan Swift - enjoy for the story or read a little bit deeper for a savage critique of society.
Huckleberry Finn/Tom Sawyer by Mark Twain - they aren't complicated books but very enjoyable.
Dr Jekyll & Mr Hyde by Robert Louis Stevenson - an excellent story considering the duality of human nature, you could read most of RLS's books though and enjoy them.
White Fang by Jack London - powerful story about a wolf.
The Thirty Nine Steps by John Buchan - great yarn about a spy, ignore the slight racist connotations to some phrases and accept that they were a part of the time.
Around The World In 80 Days by Jules Verne - foolishness, daring, triumph all in one book.
Lanark by Alisdair Gray - breathtaking vision, amazing, a novel in four parts as it says and what parts they are.
I've missed some obvious ones and I've included some that are perhaps less known or less accomplished than others but they are all books I've enjoyed.
//edit for more books.
The Master And Margarita by Mikhail Bulgakov - it's simply a great satire.
The Three Muskateers by Alexandre Dumas
Othello by Shakespeare - you should read at least one of his and it might as well be this one.
Watership Down by Richard Adams - more than the sum of its parts, for a childrens novel it is remarkably hard hitting.
Ender's Game by Orson Scott Card - great science fiction.
Wiseguy by Nicholas Pileggi - the book that Goodfellas was based upon
The Godfather Trilogy by Mario Puzo - an epic, need I say more.
I could probably go on all morning but realistically I'd say just get reading, if you allow yourself to read widely enough you'll enjoy books that I wouldn't even think to suggest.