This question is quite subjective and is usually met with the reply, "it depends on what you want to achieve".
If it's cardio fitness that you are looking for, then the more intense the exercise the more benefit you are likely to receive. Whichever piece of equipment you choose to use, you will only get out of it what you are willing to put in. But there are some exceptions, for example it's safe to say that using a rowing machine will be harder work than pedalling an exercise bike for the same amount of time, therefore burning more calories and having to work at a higher heart rate level.
If building strength is your goal then using free-weights rather than a multi gym at the same resistance level will be harder, thus creating more muscular fatigue.
But having said all that, it can also depend a lot on the user themselves. Performing the exercise movement in the correct manner, with the intensity needed can be a large factor. You may have one person doing everything right who can get more out of the exact same exercise than someone else who has not practiced the movement, nor understands why or how they should be working out.
Those looking for a definitive list of "the most effective gym equipment" will struggle to find one for these very reasons, but very loosely put and assuming that the same person tried them with correct form and the same intensity under the same conditions then it might look something like this;
for aerobic work;
and for anaerobic work
Not that the least or lesser effective methods are no good, just that they may not be as good as the others for what you want to achieve and this is a very important point to understand.
To finish - you may also find that all of the above may be the complete opposite for you. If you were to go for what could be perceived as a more effective method of exercising but find it too difficult to use then there is a high likelihood that you wouldn't not carry out the exercise in the correct manner or worse, give up on it completely. This then makes it the least effective equipment of all.
The take-away.
You need to decide what works for you, everybody is different. By picking the equipment that you can use regularly and that you won't get burnt out on or bored of will be the most effective you can buy, whatever it may be.
