Pikachu, Jolteon, Pidgeotto, Glaceon, and Vanilite, are out of the question on the competitive scene.
Despite Jolteons incredible speed, it lacks a good movepool, and it has frail defences
Pidgeotto is unevolved and won’t have access to moves Pidgeot has, such as Heat Wave.
Pikachu hasn’t got much either, even with a Lightball to boost its Attack and Sp attack, but is preferable over Jolteon.
Glaceon has a similar case to Jolteon, high stats in special attack and defence, but it’s movepool leaves much to be desired and lack of HP or speed
Vanilite isn’t even at it’s second stage, so that’s a big no, as a lot of unevolved Pokemon have very low stats.
Most legendaries or mythical Pokemon are usually in Ubers, making them great competitively. Try to avoid unevolved or not fully evolved Pokemon, as their evolutions can do better. But this doesn’t always apply to NFE or unevolved Pokemon
