Pre-sale is great if you really believe in the project and can get you big discounts. Generally the earlier you invest you can a better deal.
Investing right near the end of the ICO or after a few days can allow you to gauge how successful it will be and make better informed decisions.
Investing post ICO can reduce the risk of getting scammed and sometimes allow you to pick up cheaper coins, conversely you can miss the opportunity to profit as most of the best ICOs will trade at a premium when launching on an exchange compared to the ICO price.