SPG may be 'most popular', but I am currently very high on the Citi Thank You Rewards Program. Works best if you fly a fair amount and want to redeem a fair amount of travel - a combination of an AT&T Universal Rewards Card plus a Citi Premier Pass Elite Card will get you a net 2-3% on all purchases and the ability to spend those points at at least 1.5-2 cents each. Net cash back equivalent on all spending is nearly 5%, which beats just about anything out there. The big advantage over SPG is the points are spent for travel directly with TY Network's travel agency, thus no transfer to an airline is required, and you don't have to deal with all the limited award availability and various rules of airline award redeptions.

There are several extensive threads about this program in this forum.