The ones that I always tend to forget about are (and yes, I had to look up a list of Pokemon because I neglect them that much):
Slugma/Magcargo
Corsola
Tyrogue/Hitmontop
(Lots from Gen III... not my favorite)
Lotad line
Wingull/Pelipper
Shroomish/Breloom
Whismur line
Nosepass/Probopass
Meditite/Medicham
Barboach/Whiscash
Chimecho
Clamperl line
Deoxys (I never obtained it back in the day, and I started playing R/S a couple years after its release)
(Gen IV is also not a favorite...)
Burmy line
Cherubi/Cherrim
Glameow/Purugly
Chingling
Bonsly
Finneon/Lumineon
Mantyke
Phione
Manaphy
Gen V is still pretty fresh in my mind, but I can see myself forgetting the Sewaddle line, Petili/Liligant, and Alomomola.
Slugma/Magcargo
Corsola
Tyrogue/Hitmontop
(Lots from Gen III... not my favorite)
Lotad line
Wingull/Pelipper
Shroomish/Breloom
Whismur line
Nosepass/Probopass
Meditite/Medicham
Barboach/Whiscash
Chimecho
Clamperl line
Deoxys (I never obtained it back in the day, and I started playing R/S a couple years after its release)
(Gen IV is also not a favorite...)
Burmy line
Cherubi/Cherrim
Glameow/Purugly
Chingling
Bonsly
Finneon/Lumineon
Mantyke
Phione
Manaphy
Gen V is still pretty fresh in my mind, but I can see myself forgetting the Sewaddle line, Petili/Liligant, and Alomomola.