I like them all. Mostly Pikachu because it can still learn useful moves before you evolve it into a Raichu. After Raichu, you can only rely on TMs, and it's best to keep most of those moves. Especially Agility.
I'd choose Pikachu in this case. For Pichu, I didn't really like him because he didn't have many good moves at all to use, and it would be a pain trying to get him to evolve, since it required having his love at the max, and I never liked Raichu mainly because of how I thought he looked pretty ugly, Nya~.
I choose Raichu, because that one is my favorite partially, but also because if trained right Raichu can easily defeat a Pikachu, and Pichu shocks himself when it uses an electric attack that seems like a pretty big flaw to me if you are an electric type Pokemon! XD