The simple answer here is going to be whatever you adhere to consistently. If you like mud runs, crossfit, yoga, jazzercise, powerlifting, cross training, mountain biking, etc...Whatever you consistently do is the best style of workout.
At the studio, most goals are centered around improving physiques, weight loss, muscle gain, lifting performance, etc. I would say the most efficient style of training for these goals would be progressive overload. In short, this is methodically making your training slightly harder week by week to provide consistent progress in strength and/or stamina.
With this style, you repeat the same exercises week after week and work on progressing the volume or amount of weight lifted. In doing so, you get more comfortable and technically sound with the lifts vs doing random exercise movements each time you work out. I find this to be more efficient, more measurable, and generally superior to all other methods of training with the assumption adherence/boredom is not an issue.