Brad Pitt says being a dad is the best thing that has ever happened to him.The 'Ocean's Thirteen' star - who has four children with partner Angelina Jolie - has spoken of his joy at being a parent. He said: "The first scan is mind-blowing. I love being a dad - it's the best thing that ever happened to me."Brad and Angelina have three adopted children - two-year-old Zahara, Pax Thien, three, and five-year-old Maddox. Angelina gave birth to the couple's daughter Shiloh, who celebrated her first birthday on Sunday. This is not the first time Brad has expressed his parenthood delight. He recently described having kids as his greatest achievement, saying: "You know, you can write a book, you can make a movie, you can draw, paint a painting, but having kids is really the most extraordinary thing I've ever taken on."The actor also joked he and 'Ocean's Thirteen' co-star Matt Damon spent most of their time on set discussing their kids.