United Airlines has long been a favorite of mine. My reasoning is anecdotal. My neighbor is a United pilot and I respect his skills as a pilot and his choice of an airline to work with. I have had relatively good luck on United flight schedules. Plus they have never left me sitting on the tarmac for hours while they fix the plane. The crew on United flights seem very nice compared to other airlines...like they really care about me.
Now, the intensive cleans are done every 30 days for the smaller United jets that fly within the and every 15 days for the wide-body aircraft that cross the Atlantic and Pacific Oceans, enduring wear and tear during the long flights that can cause planes to become run-down more quickly.