Plusle and Minun
Ehhh...well...the idea of pokemon species related to other pokemon species without being evolved forms or version counterparts is an interesting one. I actually find it kinda neat that Pikachu was expanded into a sort of "genus" in the Pokemon world, and the concept of a "positive" and "negative" electrical rodent duo isn't bad either. On the other hand, we all know these two were conceived partially to sell merchandise, they don't evolve, and their only purpose in-game is to show off the double battle feature, which I just never got into.
Kinda forgettable?