cuzant said:
Fluorine. The most reactive element, toxic gas, forms very strong acids (corrodes glass, quite easily) and its compounds have a habit of spontaneous combustion.
Eh? Hydroflouric acid is the weakest of that group. It goes HF < HCl < HBr < HI (It says Hi, I like that acid) < HAt (This one is theoretical based on trends, At is too unstable to actually test it.)
Also, it's not so much the most reactive as it is the most electronegative, there is a difference. That said, Fluorine is pretty awesome, if only for the aforementioned electronegativity.
OT: It's hard to pick a favorite... I guess I'll cheat, I'm a fan of Copper's group. I find it interesting that Silver (Ag) and Gold (Au) have been popular elements and used as currency, and not only are they in the same group, but that group has interesting electrical properties (The group has Xs[sup]1[/sup](X-1)d[sup]10[/sup] Where the s group usually fills ahead of time.
(Erm... I'm a chemistry major... I like this stuff...)