I'm a fitness instructor and personal trainer, and honestly the best nutrition comes from clean, healthy food. It takes planning and forethought, but it's so worth it.
But there are some supplements I use. I take a multi-vitamin (Sentrum for Women), and I drink a bit of chocolate milk before a strenuous workout like RPM cycle or TRX. The whey protein in milk acts fast, giving you an energy boost either before or after a workout.
If you're an extreme athlete or a body builder, additional supplements can help.