The healthiest foods are those that are processed the least.
- fruit and vegetables
- nuts and berries
- milk and cheese
- wholewheat pasta
- wholegrain bread
It's important not to ignore meat in your diet, but most people eat far too much (it sits in your gut, slowly putrefying.... yuck!). Lean chicken or beef once a week is about right.
I've done the math and found that eating far down the processing chain is cheaper than buying (coupon-reduced or otherwise) food.