I guess what we're looking for here are Pokemon with completely unremarkable characteristics, or at least characteristics shared by others. Plenty of your pure Water types fit this bill (Kingler), as do pure Normal types (Delcatty) and Bug/Flying types (Mothim).
The most forgettable Pokemon of all time could be Silcoon, who has no interesting moves or abilities, a pathetic evolution, a gimmick (cocoon) that was covered twice in the very first generation, a Pokemon that looks almost exactly like it (Cascoon), a boring typing... yeah I could go on.
Sometimes the Anime saves Pokemon falling into obscurity too, like Chimecho and Carnivine.