My favorite is a tie between Giratina Origin Forme, Deoxys Normal Forme, and Rotom Lawn Mower Forme.
Giratina Origin Forme: I was a fan of Giratina, but it didn't impress me compared to the likes of Dialga and Arceus, so I considered it to be the worst of the 4th gen legendaries (Ommiting the trio and cute legendaries). When Platinum was announced and Giratina's new forme was first heard of, it didn't impress me, either. It wasn't until they showed it's Sugimori Art where I was almost obsessed with Origin Forme's awesomeness. They somehow made an semi-impressive pokemon into an awesome one, and thats one reason why I really like GOF ( After re-reading this, I thought this reason was odd).
Deoxys Normal Forme: Deoxys was once one of my favorite pokemons, mostly becuase of how unique I thought he was when I first saw him. Then came his formes, which I thought were well-designed and gave me another reason to like him. When I saw the 7th movie, Destiny Deoxys, I was happy to see his other formes used, even if it used them rarely. When Speed Forme was introduced, I thought it was the best Deoxys Forme yet, but my interest in Speed Forme was waning when I realized that I liked Normal Forme best.
Rotom Lawn Mover Forme: I can't make this long, so i'll probably be atleast 2 sentences.
I'm wasn't a huge of Rotom until it's new Forme's were heard of, and Lawn Mover was just the one that I thought looked the impressive (Considering that it looked horrible in it's silhouette forme).
That should cover most, if not almost all of my reasons.