It might have come as a surprise in my previous post that I did not mention any animal rights theorists in my discussion of stand-out philosophers. In my defense, I wrote that post in quite a state of pain and illness, having slept most of the evening, and I needed something to do in bed while I was unable to fall asleep. It's a small miracle the post is coherent as it is.
That being said, there are some other, more interesting reasons why I didn't mention any animal rights theorists in that post. The first is that it was a question about the best philosopher, rather than best person. And you don't have to be a great philosopher or thinker to support animal rights, just as you don't have to support animal rights to be a great philosopher. Of course, the ideal philosopher ought to support animal rights, but I only considered philosophers who actually exist or existed, and was limited in that way.
The truth is, the vast majority of philosophers (and people) haven't supported animal rights, and if part of the question is who influenced the history of philosophy in important ways, that biases us toward the past when there were even fewer animal advocates
Another thought is that, as I've said, the arguments for animal rights are not too complicated, you don't have to be a brilliant philosopher to make them. Gary Francione, for example, I think is a wonderful animal advocate, and a brilliant scholar and orator, but his philosophy is not overly complex or challenging. I think he would agree . His arguments rely on a lot of popular premises, and he writes in a way that is accessible to a large audience. Which is what you do if you want to be a great advocate, not necessarily what you do if you want to be a great philosopher.
If you're going to be advocating against prejudice, you want your arguments against that prejudice to be your centerpiece. Christine Korsgaard (whom I did mention in that post) in fact does include animals as a significant subjects of moral obligations in her work, though is somewhat vague on the precise prescriptions that follow. But many elements of her work are very controversial(and expertly argued), so if her aim was to be an unabashed animal advocate, I'd recommend losing some of the Kant-talk. Because she's interested in being a great philosopher, she writes a lot about Kant.
Finally, there is something of a bias these days against professional philosophers doing advocacy work. I get the impression that many think it unseemly, that it's more appropriate to write about things like moral worth and blame in peer-reviewed journals, and less appropriate to be telling other people that their actions are wrong. This is one part of academic philosophy that I see as very problematic, and I worry that many great voices for change might be wasted in echo chambers. But too, I think I carry some of this bias, and thus fail to see outspoken advocates as less philosophically "pure" somehow.