Let's see... for me, personally, almost anything found in a cave.
The first few times I played red, I hardly did anything with Machop, Onix, or Zubat. Even Clefairy, which has great potential, got fairly neglected.
I did use Dugtrio a good deal, since it learned Dig without a TM, and when I was using Squirtle as a starter, Graveler was a must-have against Surge. Silly lightning types couldn't do a thing to him! Made me feel powerful.
I once got a Beedrill's level up in the 50's, but I never found it that useful in the later stages of the game. Same with Butterfree.
For me, I'd say Jigglypuff is the most forgotten Pokemon. I've never gotten one past lv. 15 or so.