This is the only best friend on the list who is literally created by the main star. Elsa uses her magical ice powers to make the perfect companion for her and her sister, Anna, when they are kids. When they grow up and Elsa storms off and makes an ice palace in the forest, she recreates Olaf. He helps Anna find her sister and makes her laugh along the way. Later, he tells Anna, "some people are worth melting for," when she is worried about him sitting too close to the fire — which he was only doing to keep her warm and protect her from dying. He even tells Anna that the charming Kristoff is in love with her, helping her realise her true feelings for him as well.
He becomes such an important part of the story that Elsa creates his own personal snow flurry at the end so he can stick around through every season. His positive attitude and hilarious jokes make him number one on my list!