Most energy-dense commercial battery available
This has a list of common battery types with energy densities.

Lithium ion primary batteries appear to have the highest specific energy at around 300 Wh/kg. Lead-acid batteries have a specific energy around 10% that of lithium-ion batteries, but are extremely reliable (depth of discharge, self-discharge, etc.), and do not have nearly the same heating/fire issues (though they can explode if you mess with them), which is one of the reasons they have endured so long in automotive applications.

More exotic chemistries have higher specific energies, but I am unsure as to whether they are available to the average consumer.