Phaser. With the caveat that it won't have the same settings as its Trek counterpart. Some country somewhere (other than the U.S.) is going to stop caring about what the U.N. considers a war crime and start constructing man-portable Free Electron Lasers with variable frequencies. Infrared for cutting, visible/utraviolet for blinding, and then dial it up to x-rays and/or gamma rays for use against tanks or APCs. Larger version mounted on a naval vessel or an airplane would make for a poor man's nuclear deterrent, you could irradiate entire cities and then sit back for a couple of days and wait for everyone to die.
After much deliberation I went for phaser. Even though we have 3D printing and the beginnings of lab grown meat/organs, replicators like those in trek are a long way off yet. The only country so far to experiment with laser tech is the US. They already have on mounted on the front of a jumbo jet that can nail ballistic missiles and aircraft from many miles away. I think a man portable device is a few decades away yet. The power required to blow something up with a laser is enormous. As far as I'm aware there are no laws governing the use of laser based weapons in warfare as the weapons do not yet truely exist in a capacity where they can be used in open warfare.
^ Actually, China's been developing laser dazzlers since the mid 1990s. They're not designed to blow things up per se, mainly to emit high-powered laser beams that cause permanent damage to enemy sensors and/or vision. It was mainly intended as a sneaky way of shooting down enemy helicopters by blinding the hell out of the pilot or at least scramblig his targeting system, but they have since retired it from use under the auspices of the U.N. Incorrect. Land-based lasers are effectively restricted by U.N. conventions to be used ONLY in an anti-material role where there is no risk of inducing permanent blindness on enemy combatants. That is to say, any use of a laser that might cause eye damage constitutes a war crime. On some level this is a little silly, because a man caught in a laser weapon powerful enough to be lethal probably isn't going to live long enough to worry about his eyesight. On the other hand, a laser that ISN'T powerful enough to kill him can still permanently blind him, and that sort of weapon would be cheaper to develop and easier to deploy anyway. So the convention is, it doesn't matter if the laser weapon also burns a grown man to a cinder in three seconds flat, if it can cause blindness, it's illegal to use against human targets. This, by the way, is part of the problem with the Airborne Laser. It has to be powerful enough to shoot down ICBMs because there is no other target it can legally engage (other than maybe UAVs) without committing a war crime. As it stands, it would ALREADY be a fairly effective close air support weapon, using that laser to burn down entire columns of enemy troops, armored vehicles, bunkers, etc... but ask the Pentagon why that functionality isn't included in the design, they'll usually just change the subject.
^ Really? I had no idea! You learn something new every day. I'm pretty certain someone will find a loophole in the clause someday, weapons manufacturers are huge fans weaselling there way round international law. Just look at the devastation thermobaric weapons have on the human body (it basically turns you inside out). How different are lasers and phasers? If someone did invent phaser technology and it was far enough removed from laser technology, would it then be legal to use?
Or we'll just get another neoconservative "I can has geneva conventions?" administration who doesn't give a damn about war crimes and goes and does it anyway. OTOH there are rumors that the Israelis have been using laser weapons against Hamas for the past two years now as a kind of anti-terrorist sniper weapon. I don't know if there's anything to it or not, mainly a series of inexplicable fires and some confused reports about bright flashes of light coming apparently out of nowhere. With the Israelis, you never know... Well, it would essentially have to be a type of directed energy weapon that doesn't emit visible light. Basically, a laser emitting radiation in the far infrared range or (alternately) something in the x-ray/gamma ray spectrum. That wouldn't have alot of the same effects as a laser, but it might have some other interesting effects. Something powerful enough to act like a phaser would create a visible flash in the air that might resemble a laser beam, but it wouldn't have variable settings, it would pretty much cook you where you stand.
After stumbling upon an article about long term missions in space I may well have to revise my original vote on 'what's next' to be invented. Force fields/shields could well be along before long in a very basic form of radiation deflection for spacecraft. I'm not sure if there's anything about at the moment but 'electromagnetic shields' have been postulated as feasible in the near future. Im not sure how closely these can be compared to the ones we see in trek but maybe the 1st stepping stone on the way to them.
^ There's a guy in Russia working on an electromagnetic heat shield for space craft reentries, but I doubt anything will come of it.