What island provided the most vital air base in the US island hopping campaign?

The islands and bases seized in the November 1943-February 1944 Gilbert and Marshall Islands campaign were key to establishing airfields that would allow land based air support for the upcoming operations across the Central Pacific. The campaign began with a costly three-day battle at Tarawa, but it's seizure gave the Americans the air control that enabled them to win the Marianas Campaign the following year, and begin daily bombing of Japan.