Go to this address and see all the current kind of back-up sump pumps that are made. http://www.zoeller.com/zcopump/Products/backupsystems/backup.htm
There are variations made by others. Your question was “what is the best” will be defined by what you want to spend. The higher the cost, the better the system. Any pump manufacturers system will require on going maintainence of the battery, or the testing of the switches / valves, etc. on water pump back-up units.
[Edited by John Aldrich on 18 August 2001]