Usually Helium, though during world war 2 the Germans used hydrogen because they could not get access to helium which at the time only came from the U.S., I think that is still the case. read more
Helium is a Nobel element so it does not react with anything, so it non flammable. But because it's does react with anything and it's lighter than air it's hard to get, the sources of helium here on earth are from natural radioactive decay in the ground. read more