For anyone who hasn't read any of his books, I'd recommend The Crow Road or Espedair Street as starting points (I've not read many of his Culture books I'm afraid). The Wasp Factory is his first and the first I read by him but it's perhaps a little too dark for some and might put new readers off.
The Bridge is also great (once you're into him). I'd perhaps avoid A Song of Stone, The Business and Canal Dreams as none of these are his best works.
Still, he was a great author. He will be sorely missed.
http://chrishallamworldview.wordpress.com/2013/06/25/iain-banks-where-to-start/