Samus, or really any character who just sits back and camps with projectiles. It's just such a flowchart-y, cowardly style of play that I can't stand to play against. I find that the only way to make those types of matchups not a slog is to go a character who can camp yourself, which just amounts to lobbing projectiles back and forth until someone cracks.
Also, swordies in general, but especially Marcina. I just have no idea how to get a hit on those ****ers. Any time I think I'm within attacking range, they just slap me away with any of their character-sized hitboxes, and I can't really lame them out either since they're so fast. Where does Marth get off, anyway, being top tier in every game?