as shown on mythbusters
mentos - nucleation sites, the surface of mentos is very rough thus providing innumerous surface area for carbon dioxide to react. Note that fruit flavoured mentos will not work because the surface is waxed.
diet coke - mainly due to carbon dioxide, but caffeine and aspartame are 2 of the active ingredients behind the explosive reaction. plain soda water and non-diet soda drinks don't work as well.