Hands down, the most oft-cited example of a useless Pokemon is Magikarp. I mean, it's definitely not one of the more badass Pokemon. Stupid fish. All it does is splash around, right? Well, until it decides to splash right over the top of a friggin' mountain.
Upon a bit of analysis from Youtuber VSauce3, Magikarp could certainly turn an unsuspecting victim into fish food. It has the ability to leap over a mountain, and with it weighing in at 22 pounds, that means immediate death for whomever it lands on. VSauce3 gets real sciencey with it, and eventually equates it to being strapped to the front of a cannon. Of course, going out by way of a cannon would be far less awkward for your loved ones to explain than getting fished to death.