I decided that I wanted to be a voice on every animated cartoon in the history of the world - even shows that haven't been on the air for a very long time, that's going to be harder to pull off.
It's hard to really articulate what the parameters are that make one song parody-able and another song not, but if I can come up with a good enough idea for it, I go for it, and if not, then I have to move on.
Not any specific one, but I was a huge fan of Frank Jacobs, I guess he wrote the plurality of the song parodies for MAD, Sam Hart, a few others, but that was also where I was first exposed to the art form of song parodies.