Which form of weight work is most effective?
The primary lifting disciplines are:
- Machines
- Free Weights (barbells & dumbbells)
- Cables
My canned answer is “depends on your goals and level of experience.”
Many women and older people tend to use machines because of the ease of use and safety factor. The safety factor of machine systems will provide a mechanical stop; however the strict and controlled movement also limits the recruitment of other muscle(s) groups during the actual movement. A bench press with dumbbells is a primary example of various muscle groups not only being employed to raise the dumbbell, but also using other muscle groups (e.g. shoulders) to keep the weight in balance to maintaining control of the weight.
The leg curl and leg extension machines on the other hand and are much more effective for lower leg work. The machine cam type design load the quadriceps and hamstring in a way not possible with free weight loaded machines of lesser design. Consider the amount of you can push with a leg press or hack squat: 400, 500, 600 lbs and above. For the vast majority this amount is completely unreachable in a free weight barbell squat, but once again the free weight squat offers extreme muscle recruitment for the balancing and control of the weight.
For the development of sheer lifting power the barbell is the reigning power king. Consider the Olympic lifts of clean and snatch, two hand clean & jerk, squat, and deadlift…raw sheer power and a tremendous amount of strength has been developed to move the weight. For further reference on Olympic lifting see: http://www.bodybuilding.com/fun/schultz26.htm or http://www.muscletalk.co.uk/article-olympic-lifting.aspx . These ladies and gentlemen use barbells to train and develop the strength and power to move extremely heavy loads.
Dumbbells are effective for developing balance control, muscle recruitment and cutting up (greater muscle definition). 
The use of dumbbells requires a greater degree of experience not only in their respective handling but in knowledge of targeted muscles groups.
The cable systems (often overlooked by many) in my opinion provides the greatest degree of targeted muscle development. With a cable system the muscles are constantly over loaded during the entire movement phase. Both the positive and negative repetitions maintain a load on the targeted muscles groups. Associated muscle groups are also recruited during the movments. An example of a cable movement would be a lat pull down, cable curl and a tricep extension.
The following is a breakdown of systems advantages and disadvantages:
Machines:
Best safety factor and ease of use
Poor muscle recruitment and balance development
Barbells:
Best power development and mutual muscle recruitment
Worse for safety factor
Dumbbells:
Very good for muscle isolation, definition type work and muscle recruitment.
Medium for devloping strength
Poor for safety factor as no cage or supports can be used
Cables:
Very good for muscle isolation, definition type work and muscle recruitment.
Low for devoloping strength
Very good for safety
Be safe, be strong, but above all be smart!
