My favourite band has to be New Found Glory. I haven't been a fan from the start, I haven't even listened to their first album but I have listened to the albums released after that (except International Superheroes of Hardcore) and I am a real big fan of them.
Same to Kill Hannah. I got their first album years ago and I was just a little bit of a fan and then I listened to their second album and became a bigger fan. I even have the band logo (a heart with an aimy style thing on it, I cannot think of the word) tattoo'd on me.
I have seen both bands about 3/4 times each and don't think I would ever miss a concert. As for my favourite song, there is way too many.