"A", "To", "His", "I", "Me", "I'm".
That's my literal answer. I opened a few main forums in the General Discussion section and sorted the topics by replies, selected the first topic with the most replies, expanded it to 'all' view, and used a tool on a website to approximate what the most commonly used words were. I had to filter it a little, because the first words it read were things like the year, 'profile', 'report', 'spam', etcetera, because those words are generated by the forum and attached to every post--but excluding those...)
Oh, and most common word on the "What foods do you ABSOLUTELY hate, hate, HATE!!" thread (543 replies) was "Fish". Strangely, "Fish" appeared more than "Food".
But honestly, I'd figure that "Fat", "Muscle", and "Exercise" will be in the top words list.