Sliced bread may have been the best thing up to 1928. But as AA points out many better inventions since then such as the ipod.
Prior to sliced bread anyone could cut bread with a knife and get the same thing. Prior to the Ipod, it would be physically impossible to carry thousands of songs with me anywhere I went.
My vote goes to the Ipod or mp3 player as the best thing since sliced bread.
Edit to add: probably the smart phone would top the ipod since it can play Mp3 plus much more.