Wrath, my Ancient Grimtotem Spirit Guide.
Sure, it shares a model with Karoma now but mine isn't a Spirit Beast, it's a wolf. I think it means so much to me because I can't even remember how many times I had to try to tame it before I finally got it. There were so many times when my Mystical Skyfire Diamond would proc but my friends would forget to use their Drums of Battle and Heroism. When I finally got him, I was so happy that I started taking him with me everywhere. 5 years later and that's still the case, even if he might not be as useful as some of the other pets.