The one single best exercise that activates the biggest muscles, those in the buttocks, back and legs, are 'squats'. Squats also have a real strong aerobic component to them making the heart work harder and faster. Having said that all exercises are good, some are just better suited for different individuals, it's a choice.
It's a very simple exercise, just fold your arms across your chest, bend your knees and lower your body until your thighs are about parallel to the floor. If you've never done these before then start slowly squatting only half way down until you develop a rhythm. My experience is that the first squat or two are the most difficult, so don't get discouraged they get easier as you go.
Keep it Simple, Consistent and Fun.