My GI used to use Golytely, which is certainly effective if only due to the volume. In recent years he has switched to a much more simple and tolerable mixture:
4 Dulcolax pills at mid-afternoon the day before
A Dulcolax supository that evening
One small bottle of magnesium citrate at 8 p.m., which runs through by midnight
A second bottle at 4 the next morning, which is pretty well through by the time you leave for the appointment
Phayzyme tablets to control gas that morning
Always had a clean colon with that regimen, and aside from some mild cramping when the Dulcolax kicks in and four of five trips to the commode for each MagCit bottle, it's pretty tolerable.