I've researched into lasers, plasma weapons, sonic weapons, basically everything I can think of, and I can't find any kind of energy weapon viable as a hand-held or even vehicle mounted weapon. Lasers can be easily reflected, they have to be held on-target to inflict damage, they can be nullified by something as simple as a dust cloud or a mirror, and their power-output ratio is ridiculous for a mounted or hand-held weapon. Plasma have very similar problems, but mainly power. I don't want my soldiers each carrying a nuclear powerplant on their backs. Is there, even a theoretical energy that can be fired as a beam or projectile and preferably not have to be held on-target for more than a few seconds?
If there really is no alternative without unobtanium then I'd accept something that can fire a projectile at railgun or better speeds, if that means a power-efficient railgun then that's fine.
Edit: This is pretty far beyond our tech, while I'd like to keep the theoretical as human these weapons will be landing in alien hands.