Raichu and NuRaichu are probably my favs, but I don't count raichu for the question posed in the OP since he was never pushed as an "other" pikachu.
Pichu was despite being baby pikachu, and is the fucking best. Raichu is cool and all but pichu is the goat electric rodent mascot and is probably the only one pushed even as remotely hard as pikachu is. Fucking love that little nerd.