Rival is the originator of the Crock Pot and I think that is a pretty safe bet. They have the copywright on the name so they have a vested interest to make sure thier product works the best it can. They also make several different types.
Any other machine is only a "slow cooker" and some companies knock them off very cheeply. Any reputable company should be ok, but beware of any either unbranded or with unknown name. I have had a Sears brand with much success, and a couple of kinds of Rivals plus a Nesco 18 quart (with removable liner) that doubles as a portable oven (I can cook a turkey in it or simmer a soup or stew all day) and I love it when making larger quantities. The one I am using mostly (size wise) now is a "crock watcher" by Proctor Silex.
First decide on the features you want. I've seen one that gives a couple of different sizes of inserts (interchangable) I have a casserole one that is long, shallow and rectangular. I have a larger oval one that is 6 quarts.
When you decide on the features, make sure look for the name of a reputable manufacturer.
Edited by: LITTLEWIND53 at: 1/23/2010 (10:14)