Gyarados. This is the single best Pokemon for any in game playthrus especially in nuzlockes due to how much value it offers and said value often eclipses everything else in its area. Magikarp is a very common Pokemon in the games it is available in, except for bw2 I guess, and that's why Gyarados is just that good. It has intimidate which is often considered as the best ability in the game, a resistant latent typing, and high stats. It can even easily keep up late game because it learns dragon dance naturally. Nuzlocke or regular playthru, Gyarados is often the best Pokemon
When in doubt, use Gyarados