Oʻahu carries about 70% of Hawaii's population and nearly all of its urban energy. Some travelers write it off for that reason. They are wrong. No other island gives you this range: Pearl Harbor and ʻIolani Palace in the morning, a north shore sunset with nobody around you, then a dinner scene the neighbor islands cannot match.
It is also the easiest island to arrive on. Flights are cheapest and most frequent, hotel supply runs deep at every price point and it is the one island where you can manage a few days without a rental car. First trip to Hawaii, or a group who all want different things? Start here.