We all have at least one Pokemon we forget exists; there's over 700 now, with more to inevitably come. It's no surprise a few slip through the cracks into obscurity. They can be cool, cute, great in battle, whatever-- fact is, these Pokemon just aren't going to be the first that come to your mind, whether you like 'em or not. In other words, not "Forgettable Pokemon", but Pokemon you just don't remember.
For me... I'd say it'd be Exeggcute and Exeggutor, except I've forgotten them so many times that they're the first I think about whenever I think about which Pokemon I've forgotten. I actually think they're cool, too, and wouldn't mind giving Exeggutor a shot some time. Other than that, nothing is coming to mind... which probably means I forgot them, haha!
How about the rest of you?