I'm probably mistaken, but a lithium-gold battery would be the most effective.
I get this idea from the reactivity series, in which Lithium is the most reactive, and gold the least. A lithium-gold battery, then, would have the most energy possible. The series also implies the possibility of electroplating, with low reaction things being easiest to electroplate, with gold plating the easiest and lithium plating being impossible.
On the impractical side of this, gold is a rare material enjoying great demand for its non-corrosive nature, electrical conductivity, and good looks for jewlry. Gold is expensive, making such a battery impractical.
Ah well, it was probably a stupid idea to begin with.