Lately I begin with arpeggios to a drone, and move the arpeggios in fourths. I set the drone a 4th above the tonic (if I begin with a G arpeggio then the drone is a C) and don't change the drone until I've worked through 3 keys. For example, starting on G I would play the arpeggio over the full range of the bass, but set to a C drone. Then I would do the same with a C arpeggio, still set the the C drone. Next I would do F to the C drone. After that I change the drone to Eb, and do the same pattern with the arpeggios, beginning with Bb. To me this is a little more fun than changing drones per key, and works my ear in different ways. I wouldn't say it is the "one," maybe just one of them. I also do other warm-ups, mostly Simandl.