1. The Plank. This is the ab exercise where you hold your body rigid and prop yourself up on your elbows. But unless you're a pure beginner, it doesn't do anything. You need more resistance to give your torso a real workout.
2. Crunches. They really only target one small part of your abs, and they can put too much strain on your neck and back if you don't do the form exactly right. So, you're better off using some kind of prop, like a balance ball.
3. The Bench Press. Obviously it works, but it's overrated. You need other kinds of exercises to get a full arm and chest workout. So some trainers recommend using dumbells instead, and focusing on inclines and military presses.
4. The Seated Knee Extension. This is when you sit on a weight machine with your legs bent, and lift weights with your feet until your leg is completely horizontal. It works your quads and that's it. You're better off doing a full body exercise like lunges or squats.
5. Hanging Knee Raises. That's where you hang between two bars and curl your knees up to your chest. This one is pretty hard to do correctly, so it's easy to get hurt. Just do bicycles instead....lying on your back and kicking as if you're pedaling a bike.