Cesium.
Because it makes things go WTFboom.
Either that or Mercury, because it's toxic and liquid.
And otherwise awesome.
Why don't you make a 118 option poll?
Make a poll with all the major elements
(Maybe gold, oxygen, helium, nitrogen, sulfur, hydrogen, zinc, calcium, carbon, silver, copper, sodium, chlorine, iron, platinum, mercury, neon, and uranium?) and then one for other.