One of the most popular collections of myth is Mythology: Timeless Tales of Gods and Heroes by Edith Hamilton. This a selection of myths, mostly from the Greco-Roman traditions, but also some Norse myths.
The popularity of this book is the way Hamilton tells the stories, keeping the original charm but presenting them in accessible language.
Both a positive and a negative is that she sometimes combines accounts from different sources when telling a particular myth. The good things is that she identifies what sources she is using.
Edith Hamilton obviously loved mythology and it shows through in this book. If you want to get the big picture of the most popular myths, this is a good place to start.
