Whats the best workout to build higher energy levels?

Increase adrenaline. Higher energy levels can be attained from regular cardiovascular (cv) exercise with interval "sprint" training. Cv exercise (with a heart rate of at least 60% of 220 minus your age) for at least 4 days a week (why 4, because then you are exercising more days than you are not) for at least 40 minutes. Two to three times during that 40 minutes, you should increase your pace (run faster, cycle faster, more revolutions per minute) and get your heart rate up higher for 1-2 minutes. These "sprints" will further raise your adrenaline level and provide you with a higher baseline of adrenaline, resulting in higher energy, increased alertness, and greater calorie burn, even at rest.