I love the BigPackets GMod hack. It may be free, but it is certainly one of the longest running and most supported hacks today.
First of all, the amount of features in the hack is immense. You can select down to bones, and the hack will work on those without question. You can tailor the hack to your exact specifications, and the hack will do the best it can to meet the specifications and still work. For example, on school shooter simulator, the aimbot is godly. Moving targets? Fuckin' easy. Hiding targets? Find em with the visual interface.
I challenge anyone to find something wrong with this hack. It's quite possibly the best free hack I've used in my entire life.