Obviously the ones menioned above are not the most forgettable, because everyone remembered them. I had forgotten mine until looking at several pokedexes and coming across the most forgettable, tropius.
i had forgotten all about the hoppip/skiploom/jumpluff line until i was playing through leaf green. i saw it and just instantly went 'OH YEAH!' because i'd always wanted to train one up to lvl.100, but never had the patience. [just a note, workin' on it now =D]