Tom Kelso: The best training modes to use, the optimal intensity to apply, the correct volume of work, the most feasible schedule, the best exercises to match one’s goal, and other factors. They must be considered for positive results. Is any one more important than another?
Fred Fornicola: All of the above are factors to be considered when developing a fitness program but the one you didn’t mention (but I know matters to you greatly as well) is safety! Safety isn’t necessarily in reference to only exercise selection but performance of an exercise, utilization 
of the correct amount of intensity and frequency as well as the proper amount of rest and sufficient nutrition to promote healing. This goes back to my above response as most trainees blindly follow along – especially if they are participating in a group class or being coached by someone individually or have read about some training ideas. They have little or no regard for the possibility that they can be injured by choosing an exercise or protocol, training frequency or some other variable that they need to address on an individual level. And keep in mind; you don’t necessarily need to get hurt in the gym as a buildup of repetitive incorrect training (on all levels described above) can lead to injury or illness outside the gym walls.
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