the best power to weight ratio is actually Zinc-air
course they aint making them in normal sizes yet.
the next best would be lithium, then li-ion.
but that is only power to weight ratio, the most consumer and easiest, and thoughless, and resonably safe , would be some kinda ni-mhy, then take the discharge problem away, and get a slightly lower capacity LSD (low self discharge) type nimhy
if wasting money were no object, then lithium primary would be the most power in a consumer battery. but li-ion is soooo close, why waste money and the enviroment?
this cell claims "UL" aproval
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1336
this is NOT protected, and if your going to use the word safe, then you should get a protected cell. it protects from over charge, underdischarge, and shorts. but can also cause shorts when damaged, due to the way you have to connect the protection up.
a LG chemical battery is what i would prefer, the ones i have tested DO disconnect on gas pressure. some of this china stuff dont.
http://www.dealextreme.com/details.dx/sku.975
slightly higher CLAIMED capacity, wouldnt trust the claim though