Technically, every Pokémon has been trained and/or used by every player of the games. Everyone has their personal preference of which Pokémon they may like to use and which ones they may not. Many like to be original with their teams and use Pokémon that others may underestimate, while some prefer to use the most common choices for the team in the games.
However, this isn't to say that there are some Pokémon who are used the least for a chosen team, such as Magikarp as some have already stated. This Pokémon may seem weak, but EV it in Attack and Speed with good IVs, teach it Flail and you'll see just what it can do. As for others, Bibarel may be one and I do agree it does learn some very decent moves. A few more to list may be Dunsparce, yet it can learn so much and be a helpful member though many prefer not to use it at times. The list is endless, but basically it is a matter of what people prefer to use, and many prefer to be original yet they choose some of the most well known Pokémon because of how widely they are used rather than those who appear to be looked down upon.
