I'm not sure what you mean by "favorite", because I don't like to see any of them. My favorite one to complain about is the appeal to nature fallacy, because it's so common. I see it constantly in commercials for foods and cleaning products, and it's the foundation of the organic food industry. The CAM field is also largely based on this fallacy. I also see it a lot when people try to justify certain ethical choices. For example, it's really common for people who think homosexuality is wrong to claim it's unnatural, and even some rape apologists will justify it by saying it had some evolutionary benefit for our ancestors. I have actually heard a few people argue that ducks rape, so it's ok for humans to do the same thing.

In reality, either everything is natural or nothing is. There's no good definition of "natural", but a lot of people consider things to be natural if our ancestors did those things. Of course, there's no distinct cut-off point, so what my own parents did might be considered less natural than what my ancestors of a thousand years ago did.

Even if we could define "natural", it wouldn't be very useful. Our "natural" state until just less than a century ago was pretty horrible and I wouldn't want to live that way. Nearly every family suffered the death of at least one child, and childbirth was a leading cause of death among women. Many people suffered from starvation and malnutrition, and they didn't have the basic luxuries that we take for granted today, like air conditioning, electricity, and indoor plumbing.

There's a nice list over at Wikipedia: http://en.wikipedia.org/wiki/List_of_fallacies which I go through once in a while. I find a new favourite every day.

I suppose I've always found the headline "50% OF PUPILS BELOW AVERAGE READING AGE!" to be quite funny.

I do like the appeal to nature one above - never thought about it until reading this - it's a good point.

Also the authority / culture fallacy is a painfully common one. Usually found in the bastions of conservative local councils where old men don't like change. Comfort in custom has its place, but not if it is at the expense of improvements.

However, I would like to repeat an answer I gave to one of ther first questions on here. the logical fallacy I really despise is the "effort made" fallacy, which goes like this:

Statement: "This project is fundamentally flawed and wholly useless." Reply: "I spent ages doing this and worked really hard on this."

This reply is not an answer to the argument - I could spend ages building a house out of sand, but in a slight wind it would be rubbish.

Appeal to nature is a particular favourite of mine - so much so I wrote a blogpost about it a while back.

The strawman fallacy is another that I find myself regularly having to expose and counter - even sceptics are quite bad for falling back on the strawman. It's so easy to misunderstand someone's argument and then offer a counter based on your perception of what they meant, rather than what they actually said. Of course, some people deliberately construct a strawman - which is always something fun to tear apart.

The False Dichotomy, closely followed by its cousins the Straw Man and the Slippery Slope.

Thinking of examples for these all of them seem to crop up in politics quite regularly. No surprises there, I suppose.