As the debate over the Indus script – on if it is a language or not and if so, is it Dravidian or Indo-European – continues, New Scientist has an article listing seven other scripts that cannot be read.
Proto-Elamite is the world’s oldest undeciphered script – assuming that it really is a fully developed writing system, which is by no means certain. It was used for perhaps 150 years from around 3050 BC in Elam, the biblical name for an area that corresponds roughly to today’s oilfields of western Iran. It is almost as old as the oldest writing of all, the earliest cuneiform from Mesopotamia. Little is known about the people who wrote the script.[Decoding antiquity: Eight scripts that still can't be read - life - 27 May 2009 - New Scientist]