The best weapon depends on the scenario:
Gamma Rays are best vs. Organic materials, and having a few minutes of this exposure, at least, helps.
A maser, microwave laser, makes a large amount of heat and is helpful vs. Cities, planets, stationary ships, etc.
In my opinion, a shotgun like scattergun that shoots millions of textbook size rocks at extremely high speeds. Hard to track, hard to defend against.
For land fighting, guns equipped with a bullet made of cesium, water inside a capsule in the bullet, and a smaller capsule equipped with millions of small, heavy, radioactive materials. Cesium + water makes a large explosion, which implants radioactive material in the person, producing both short and long term drastic effects.