the ATS forums
I've posted on several other ufo forums over the years and one has to be careful how they present their skeptical questions and positions on some of the more ufo/alien accepting boards. They tend not to like abrupt confrontation from newbies and it's better if you work your way in for a few days with apparent neutrality until you pounce on them. Even then many of the boards will confront you and ask you to not be so 'negative' and even ban you if you keep it up.
"If the human brain were so simple that we could understand it, we would be so simple that we couldn't."