Sacramento International Airport
The closest airport to Davis is the Sacramento airport, a 20 minute trip to Davis, with no traffic risk if you take the northerly (and shorter) route through Woodland. A taxi from the airport will cost approximately $40-50. Taxis will generally be waiting at the airport. A more economic option (roughly $23/person) is the Davis Airporter, which must be reserved ahead of time (530-756-6715).
Oakland International Airport
The second closest airport is Oakland, about 1 hour and a quarter away, though with possible traffic delays during rush hour.
San Francisco International Airport
A third reasonable alternative is San Francisco, an hour and a half away, again with possible traffic delays during rush hour.