For me? The Welding Torch!
I always use it! It's longer range and more powerful than the punch, meaning you can waste aliens without getting a scratch most of the time; it's the only method of getting into containers for the yummy, yummy goodies within; it's easy to make, taking only two items, one of which is obscenely common, its incredibly accurate and best of all, it's totally free! What was Ville thinking when he made such an overwhelmingly great weapon?!
Blather aside, go ahead, what's yours?