I don't know that I have a single least-favorite monster. There are plenty of monsters I generally don't use, but none really stand out for me. Overall, probably, the monsters I really don't like using most are the law and chaos aligned monsters. I'm not a huge fan of alignment to begin with, but good and evil make more sense to me than law and chaos. To be fair, I rarely use demons, devils, or angels either, but the ones I do use (like the balor and pit fiend) are more iconic and interesting to me than monsters like slaads or inevitables.
I don't really have much to say on this subject. At one point or another, I've used (or at least planned on using) most of the monsters in the 3.5 Monster Manual and Monster Manual 2.