Piplup
If tortoises are my favourite animals, penguins are definitely my favourite bird, and I waited so long for a penguin Pokémon! (People claimed Delibird was one, but... it's clearly a Santa. :P) I was delighted when they not only made an adorable one, but had it as a starter - I knew there'd be plenty of merch for me to collect. I was even happier when they made it Dawn/Hikari's main Pokémon in the TV show! Then I realised that Piplup is now almost as hard to collect as Pikachu, and it's been eating a hole in my wallet ever since...