Whoo-whee, that's a subjective area...that's also really fun! Can't wait to see the answers here.
Starting with what works (and, granted, is cliched) doesn't hurt: Striking thin metal wires or springs under tension with other metallic objects. Good for base layers and achieving the baseline Star Wars or Wall-E sound, even if you do it just as an exercise to dive deep into what a listener will expect and then branch out from there.
Beyond that, I like using FM synthesis for creating sound layers for energy weapons. It can work well when combined with real-world gun sounds or explosions, often run through effects. Subtractive synthesis is also handy sometimes, but somehow FM methods get me closer to what I like faster.
Of course, never forget to get back to the details of the weapon at hand: Many energy weapons can be described differently in terms of their mechanisms and even their visuals, and rationalizing how the device might actually work might yield interesting ideas, such as integrating the sounds of capacitors charging, flywheels storing energy, and other such hard sci-fi rationalizations. Plus, hey, it's fun to uber-nerd-out on this stuff. :-)