I'm not sure if the op is asking which Pokémon I forget most often, or which Pokémon I think Pokémon forgets or people in general. These aren't based on power or battle either.
For me the Pokémon I forget most often are Kricketot and Kricketune.
For one I can never remember which order they go in. That's partly due to the fact I never used most of gen 4's Pokémon in game, I was shocked to hear Kricketot's cry sounded like that when I first heard it recently in Battle Maison.
Some other Pokémon, I don't forget these but they seem to be not really memorable.
Finneon
Lumineon
Bonsly
Sunkern
Sunflora
Glameow
Purugly
Burmy
Wormadam
Mothim
Cherubi
Cherrim
Cranidos
Rampardos
Shieldon
Bastiodon
Bouffalant
Stantler
Deerling
Sawsbuck
Cryogonal
Girafarig
Fortunately I still bought the big guide/pokedex books even when I stopped playing the games, to keep up with the new Pokémon, there's nothing like discovering there's an eagle Pokémon or an anteater Pokémon by flipping though some pages in a book.
It's hard to list Gen 1 Pokémon in this list since if someone mentioned any one I'd know it for sure. But there are a couple that seem to be forgotten.
Seel
Dewgong
Paras
Parasect
Kabuto
Omanyte
Omastar
I think the less creative a Pokémon is the easier it is to be forgotten. There's a lot of Pokémon you can tell were just created so they could have the Pokémon types you expect in a generation, the bird, the rodent, the plant, the fossil, the fish and so on.