[Naix]

@Mekkah
Yellow Pikachu, from my experience, is similar to the Kokiri Sword in terms of usefulness. On Route 3, there's a trainer with 3 Lv. 9 Pidgeys. Pikachu was Lv. 10, and its Thundershock failed to OHKO any of them. I see Yellow Pikachu as an okay early-game Pokemon whose main use mid-game is to weaken wild Pokemon with Thunder Wave+Seismic Toss/Thundershock.
On an unrelated note, I think Ekans might have potential, as it's essenially Bellsprout-lite when it comes to Wrap, and it can learn Earthquake and Dig, which makes is a mini-Nidoking/queen in that regard. Arbok even has similar stats. I'd start a new file on Red to test it out if I didn't have a scramble file on it.
Magmar honestly deserves Bottom, in my opinion, due to the fact that it's a late-game Fire-type that only knows Ember and (possibly) Leer when you get it, making it very TM dependent. Also, for all the trouble of getting it, its stats are merely average, with nothing interesting, and it doesn't exactly excel anywhere.
TL;DR Yellow Pikachu should stay Low, Ekans has potential on paper, Magmar is terrible.

I do agree with you on Mankey, but I don't see Dig as useful on Mankey than on say, Charmander, as it and Karate Chop have the same base power. Because of that, Dig is really only useful for Electric- and Poison-types. Dig doesn't help its problem of failing to OHKO Pokemon that aren't hit super-effectively. I taught Mankey Dig in my file, and I probably should've mentioned I did, but the only reason I taught it that was because none of my other Pokemon could learn it. Overall, I stand on my tiering of Yellow Pikachu and Yellow Mankey at Low and Middle, respectively.