Even if I could only choose one mon, I would still choose Altaria. It'd be a great buddy, but it can also Mega Evolve to be a strong Pokémon, and could fly around with me if we wanted .u.
Togekiss will forever and always be my favorite Pokémon. I mean it's a giant, loveable ball of fluff. I mean read its Pokédex entry!! It's a sweetheart.
Litwick. This thing is basically my mascot at this point (at least on the Discord server). Plus, I wouldn't be in danger of it leeching my soul because I have no soul. Everyone wins! Except, of course, the ones it does leech souls from...