If you mean as a package with an adjustable output, then yeah probably is the conquest (although it still isn't exactly light)
Next would probably be the psion (sp?) from PMI then one of the preset/reg combo's like the THOR.
Otherwise just get a preset tank, lightest of all. Haven't seen them but aren't the DYE ones supposed to be titanium or something? but a preset would be lightest.
I am actually going away from adjustable outputs (and my Conquest) purely for the convenience and shape of a preset.