The question of the most useless villain is one that has people usually going to those lists of truly awful characters, like Condiment Man, Calendar Man and Turner D. Century to go on about how useless they are. Those characters tend to be more about filler stories and one-off’s just to create some kind of purchasable material, especially from the older days of comics where things were done on a shoestring and sometimes little care just to meet deadlines. There’s plenty of derision to go around for those characters, though sometimes they can be reinvented in interesting ways with some sharp writing and designs.
That said, I continue to find a different class of villain useless in the bigger picture. Those are the ones that operate more on a galactic scale, and because they do so, they create larger than life events to be dealt with but end up causing little in the way of real change when you get down to it. Having been re-reading The Infinity Gauntlet recently, it’s made clear once again that with characters like Thanos, as cool as he is and with an interesting story relation to Death herself, he continually subverts his own goals to allow himself to be defeated. When you see that going on multiple times, and in a miniseries like this where events get wiped away at the end, you end up tying useless to ineffective and without any real change from the event. Thanos certainly puts on a good show and makes for a big name villain, but how much change has he really affected over the years?
Similarly, you get much the same when it comes to characters like Galactus. While the threat is real, his role in most books tends to be more as a bogeyman in a way until a carefully designed puzzle is solved to figure out how to distract him or send him on his way. The only time that I really found him to be engaging was in the original Secret Wars series sine he was working towards the goal of defeating the Beyonder in order to gain his power to sate his hunger. That was one of the few times he felt like a truly effective character and one that worked in the background of the story for awhile at a higher level. But beyond that, he’s largely been useless when it comes to the Earth side of things. Though he’s at least caused a lot of carnage elsewhere over the years on other worlds.