The American Council on Exercise (ACE) polled 17,000 fitness professionals, asking them to name one exercise they couldn't bear to do without. Their top picks, according to rank:
1. Squats- quickly target all the major lower-body muscles and build balance and strength at the same time
2. Running- requires little gear and boosts cardio fitness and leg strength
3. Abs Exercises- are adaptable to ALL fitness levels and can be done anywhere; you don't need equipment, but doing them on a stability ball works your entire core and reduces back pain.
4. Lunges- are great all-in-one thigh, butt, and calf strengtheners that also promote good balance and posture.
5. Walking- is a simple do-anywhere-anytime leg toner and cardio booster
6. Push-Ups- are a no-equipment way to target the upper body and core
7. Yoga- promotes coordination, flexibility, concentration and relaxation
(I would say the same thing for pilates and you have to beware of the meditation part of yoga)