Pichu is my favorite character. Mario's on my main list due to him being who I would use when playing against a level 9 computer (since the only Smash Bros. fan that I know is my cousin, who in New York). Pikachu is pretty similar to Pichu, but to me, he feels weird. Ironically, mained Pikachu (I also used Captain Falcon, but his startup time was too long for me) before I unlocked Pichu, but then I got used to Pichu because I wanted to try him out, and eventually, he stuck. I then had a hard time playing as anyone else. Pikachu felt really weird, because Pikachu's moveset made me think I was using Pichu, and thus I tried playing like Pichu. The differences caused me to want to use someone else, so I picked Samus. However, I had too much trouble with using Super Missiles instead of standard Missiles, so I went with Dr. Mario. Dr. Mario's properties annoyed me (I personally found his side smash to be terrible), so I went with Mario.
I really wish that Pichu wasn't so darn light!