RBY OU What would be the best not-fully-evolved pokemon to use in an OU match?

I'm only asking this out of curiosity and I really wonder if any NFE pokemon is almost OU viable.

I guess there's two reasons why you would use an NFE pokemon
1. You already have it's fully evolved form in your team and want to have both of them
2. Sometimes it can have a better stat than the fully evolved one (for example Poliwhirl is quite a lot faster than Poliwrath)

 

Also typing. Poliwhirl isn't weak to Psychic, for instance, while Poliwrath is.

I do recall examples of NFEs being more relevant than their FEs, but aside from Poliwhirl they're all from later gens (Porygon2 vs. Porygon-Z, for instance, Pikachu with Light Ball, and the bazillion things with Eviolite in gen 5+).

 

Enigami this thread reminded me that we used to have Kadabra in F rank on the VR, which I think is pretty interesting.

Anyway, I think it's obviously between Kadabra and Haunter here, as I don't think any of the NFEs are good enough, I guess Poliwhirl would be a distant 3rd maybe? Haunter I think is better at filling Gengar's shoes than Kadabra is for Zam, since as Enigami mentioned, Kadabra's inferior bulk is a huge deal and the worse speed is absolutely brutal as well, whereas Haunter doesn't need good stats to use Hypnosis and wall Victreebel and all the random niche threats that it walls. However, I'd still prefer Kadabra, just because the things that Haunter excels at are too niche for my liking if I were to double up. Kadabra may be deeply flawed, but it's still fast and hits reasonably hard, and I could see it working if Tauros didn't exist although again, as Enigami mentioned, it's extremely difficult to justify Kadabra over Starmie

Overall, I don't think any of the NFEs are even close to viable though