i think all pokemon that have no evolution or pre-evolution are forgetable. Sometimes pokemon that got an evolution or pre-evolution in a later game are also forgetable.
Of course this does not include legendairy pokemon, and probably also not Chancy (Blissey is a great wall), Snorlax and Heracross (the one pokemon that can match Blissey when it comes to being a wall). Also Scyther and Onix are sometimes remembered, but any other solo or used-to-be-solo pokemon are forgetable.
pokemon that will absolutely be NOT forgoten are the irritating "all over the game from the beginning to the end" pokemon. Rattata, Pidgey, Zigzagoon in R/S, Poochyena in Emerald, and probably there is also an anoying "all over the game from the beginning to the end" pokemon in D/P, but i don't know him. Point is: all these pokemon appear in the very first fields you come across. The one exeption to this is Sentret. I think this makes him very forgetable.