Assuming that you want it to be man portable and with technology that could be reasonably available I think that the worst you are looking at is either: A broad focus laser that produces third degree burns over any exposed parts of the body, second degree burns through clothing and generally sets anything inflammable on fire, though it wouldn't have much range and it would be a chemical laser which means carrying around very nasty stuff to fuel it. Or something along the lines of a more powerful version of the taser type weapons that work by using a laser to ionise the air to provide a path for a current that gives the target a heart attack rather than just stunning them.

I'm pretty sure that a man portable particle beam wouldn't be much use for anything, unless of course you stretch the definition until it fits a conventional gun, which IMO is going to out perform any man portable energy weapon for a good while yet except possibly for specialised roles.