The best known circular argument is "The Bible is God's Word/inerrant/The Truth, because The Bible says so".
"Free trade will be good for this country. The reason is patently clear. Isn't it obvious that unrestricted commercial relations will bestow on all sections of this nation the benefits which result when there is an unimpeded flow of goods between countries?"
"Paranormal phenomena exist because I have had experiences that can only be described as paranormal."
"If such actions were not illegal, then they would not be prohibited by the law."
"I like vanilla ice cream because it's my favorite kind."
"Ralph Nader was the best candidate for president, because he was totally better than any of the others."
"The Cubans wanted Kennedy dead, Oswald was in Cuba, therefore the Cubans hired Oswald."
Begging (taking for granted) the question - assuming the base or principle the consequence is build on is true.
"The USonian war against Iraq is legal, because the USonian government wouldn't do anything illegal."
"The charges of physical abuse are absolutely untrue, because the police would never do something like that."
"Women have a right to choose whether to have an abortion or not, therefore abortion should be allowed."
"The abortion is immoral, because the unborn has a right to life."
"Affirmative Action can never be fair or just. You cannot remedy one injustice by committing another."
(But is it an injustice?)
Tautology is repeating things - "A is --- because A is ---"
"Sheena is a troll so everything she says is trolling"
"You have to follow the law, because it's the law."