Question: What event held by the Ultimate Fighting Championship was the largest regulated professional MMA event?
Context: UFC 134: Silva vs. Okami, also known as UFC Rio, was a mixed martial arts (MMA) pay-per-view event held by the Ultimate Fighting Championship on August 27, 2011 at the HSBC Arena in Rio de Janeiro, Brazil.  The event was the second that the UFC has hosted in Brazil, since 1998's UFC 17.5: Ultimate Brazil.  UFC 134 was awarded the 2011 event of the year by Sherdog. The Ultimate Fighting Championship III (later renamed UFC 3: The American Dream) was a mixed martial arts (MMA) event held by the Ultimate Fighting Championship (UFC) on September 9, 1994, at Grady Cole Center in Charlotte, North Carolina.  The event was seen live on pay-per-view in the United States, and was later released on home video. Michael Julian Johnson (born June 4, 1986) is an American mixed martial artist who competes in the lightweight division of the Ultimate Fighting Championship.  A professional MMA competitor since 2008, Johnson mostly competed in his regional circuit, before signing onto the Ultimate Fighting Championship to appear on "".  As of July 2017, he is #9 in the official UFC lightweight rankings. The Ultimate Fighting Championship Part II (later renamed UFC 2: No Way Out) was a mixed martial arts (MMA) event held by the Ultimate Fighting Championship (UFC) on March 11, 1994, at Mammoth Gardens in Denver, Colorado.  The event was seen live on pay-per-view in the United States, and was later released on home video. Raymond Anthony "Ray" Borg (born August 4, 1993), is an American mixed martial artist who competes in the flyweight division of the Ultimate Fighting Championship.  A professional MMA competitor since 2012, Borg made a name for himself fighting mostly in his native Southwestern United States, competing in promotions like King of the Cage and Legacy Fighting Championship. Mixed martial arts (MMA) in Ontario had been considered illegal until August 14, 2010, when the Ontario government announced that they would be moving forward to allow professional mixed martial arts in the province.  This was seen as a sudden change in direction as the government has been reluctant to allow MMA events to be held.  This paved the way for the largest regulated professional MMA event, UFC 129, which was held on April 30, 2011 at the Rogers Centre in Toronto. The Alliance MMA Gym is a mixed martial arts training facility located in Chula Vista, California and is home to a number of professional mixed martial artists who have found success in organizations such as the Ultimate Fighting Championship (UFC), World Extreme Cagefighting (WEC), Strikeforce and Bellator.  The Alliance MMA Gym is one of the top professional MMA training camps. Mizuto Hirota (廣田瑞人 , Hirota Mizuto , born May 5, 1981) is a Japanese mixed martial artist currently competing in the Featherweight division of the Ultimate Fighting Championship.  A professional competitor since 2005, he appeared in the Japanese MMA event Dynamite!!  2009 event and has also competed for Shooto and Strikeforce. UFC 129: St-Pierre vs. Shields was a mixed martial arts event held by the Ultimate Fighting Championship on Saturday, April 30, 2011 at Rogers Centre in Toronto, Ontario, Canada. Marcin Held (born January 18, 1992) is a Polish mixed martial artist currently competing in the Lightweight division of the Ultimate Fighting Championship.  A professional MMA competitor since 2008, Held has competed frequently in his home country of Poland and is highly touted by the media, often being regarded as "the prodigy of Polish MMA".  He fought in Bellator MMA from 2010 to 2016, challenging for the Bellator Lightweight Championship in 2015.
Answer: UFC 129