Actually, here’s a possibly unpopular opinion. The Dazzlings are the most intelligent and competent villians in the franchise, and don’t rank up too badly in terms of power either.
The Dazzlings aren’t a bad choice, but I think King Sombra’s competency is the one that really deserves a little more consideration. I mean yeah, his appearance in the Crystal Empire two-parter isn’t overwhelming, but give credit to the hints of what he used to be. The magical artifact that can stop him is not only hidden away, getting to it requires a pony who is (1) smart enough to even find the entrance to where he stored it (and smart enough to realize the palace looks fundamentally different when evil is present), (2) strong willed enough to resist his illusionary traps, and (3) magical enough to get around the endless stairs and other contrivances he put in the way. And it still ended up being a partial victory for him, because one of his traps ends up catching Twilight, who by that point was already a world saver several times over and one of the most magical ponies in Equestria. Had she not had a trusted companion along, he probably would have won.