Some of the best fantasy stories take you on a journey to other worlds by entering a portal. Below are some of the best examples of this type of story - enjoy!
The Chronicles of Narnia: The Magician's Nephew by C. S. Lewis (Part 1)
The Chronicles of Narnia: The Magician's Nephew by C. S. Lewis (Part 2)
Alice in Wonderland by Lewis Carroll
The Portal (Imager Chronicles) by Bill Myers
Five Children and It by Edith Nesbit