I think the #1 animal that I despise are mosquitoes, they bring nothing but disease and infection.
After that, I'd say I dislike self-aware species at their worst behavior, such as chimpanzees that are capable of consciously and willingly inflicting pain on others.
And lastly I heavily dislike a lot of our own species, people that are callous and uncaring, and people that harm others physically/emotionally without any sign of guilt and remorse, and etc. Not all persons are like that, but many are and they are worse than anything nature can dish out.