Depends on the type of post. If it's a random post then I'll choose MHO based off of how clever or funny it is. If it's a serious post, I'll pick the answer based on it's overall quality and whether or not others in a similar situation can benefit from the advice years later. Occasionally I'll give someone MHO if they have a low percentage.
Depends on the question. If I am looking for advice, it has to be most helpful or someone who backs up their opinion with an explanation. If it's not such a serious question, probably whoever makes me laugh out has something interesting to say.
I don't ask questions where I need help... so I just go by the most funny ones or where people have put most effort in it. if I have a question with 100+ replies again and I'm too lazy to go through them again just to choose MHO, I go by upvotes