There's no right or wrong answer here. It entirely depends on your play style. Raichu does the most DPS. Therefore, if your goal is to take down a flying or water unit who is defending a gym quickly, you'll probably what Raichu, as he can just burn it down quickly.
However, I'd opt for a Jolteon if you want a more versatile unit, and if you don't have very many high CP units, as Jolteon will do better defending a gym, and better if you want him to survive through several battles.
It also depends on how proficient you are at dodging. If you're better at dodging, a weaker pokemon isn't as much of a problem.