Minotaur V might become a candidate for the smallest current launcher to put a payload in GEO, but it has only launched LADEE yet. The first satellite in GEO seems to have been Syncom 1 which launched on a Delta B in 1963. There are different ways of being the smallest to consider.
North Korea has announced that they will launch geostationary satellites. How does the capability of their launchers thus far meet that ambition?