Best was educated at Tandragee Junior High School, Portadown College and studied Agriculture at Newcastle University where he played with the Newcastle Falcons academy. He returned home to join Belfast Harlequins in 2002 where he captained the team and gained his first Ulster contract in 2004. He made his debut for Ulster as a replacement at home to Munster in the 2005 Celtic League. He replaced his brother Simon as captain of Ulster for the 2007/08 season and continues to hold the position. He is renowned for his abilities as a scrummager. Best missed the entire 2009/10 season due to surgery on a chronic disc problem. Best scored two tries in 2011/12 and 2010/11 for Ulster.
He has been ever-present for Ireland, and became undisputed first-choice Hooker at the 2011 Rugby World Cup after an injury to Jerry Flannery. He was chosen to captain Ireland during the 2012 Six Nations Championship after Paul O'Connell was injured.
Best was raised on the family farm near Poyntzpass, County Armagh. He got married in Richhill, County Armagh during the summer of 2009, to a teacher at the Armstrong Primary School, of hockey and a primary seven class, name Jodie Bell, now Jodie Best. His brother is retired Ireland and Ulster prop Simon Best. His first child, Ben, was born on 23 June 2010.