You know, that's a really good question. I have given to the Childrens Hospital in the past, and breast cancer research and I just signed onto a 10$/Month donation for Aids research/Education, but I've always given the most to the SPCA.
I can't really give you a specific answer to be honest. I love animals obviously, even though I don't put them above humans or below.
I've also grown up around SPCAs, so I know what goes on there and how it's understaffed, under-supplied and below average conditions.
So if you want an answer, it just feels right, that's all.