Certainly, and as mentioned above , KCN pellets dropped into H2SO4 to produce HCN as used in gas chambers is right up there. But if you want to really want to know I submit agent VX for your consideration:
VX, IUPAC name O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate, is an extremely toxic substance whose only application is in chemical warfare as a nerve agent. As a chemical weapon, it is classified as a weapon of mass destruction by the United Nations in UN Resolution 687. The production and stockpiling of VX was outlawed by the Chemical Weapons Convention of 1993.
The VX nerve agent is the most well-known of the V-series of nerve agents and is considered an area denial weapon due to its physical properties. The median lethal dose (LD50) for humans is estimated to be about 734 micrograms [~0.75 mg]
VX can also be delivered in binary chemical weapons which mix in-flight to form the agent prior to release. [Separately the two components are relatively nontoxic.] Binary VX is referred to as VX2,[2] and is created by mixing aO-(2-diisopropylaminoethyl) O'-ethyl methylphosphonite (Agent QL) with elemental sulfur (Agent NE) as is done in the Bigeye aerial chemical bomb. It may also be produced by mixing with sulfur compounds, as with the liquid dimethyl polysulfide mixture (Agent NM) in the canceled XM-768 8-inch binary projectile program.
Well you did ask!
http://en.wikipedia.org/wiki/VX_%28nerve_agent%29