The 29th century phaser from "Future's End". For one simple reason: The beam it fires actually looked realistic! Most phasers we see, the beam appears to take a visible amount of time to reach its target. Which should not happen, because this is essentially a beam of light, so it should instantly appear between the phaser and the target without appearing to take any TIME to do so. The 29th century phaser is the only one we see in which this actually happens!