Use centralized exchange service if can't protect yourself by scammer, but the lack is the fees, your identity and sometimes they will suddenly collapse or turn into scam (ect situation).
Use p2p exchange if you can protect yourself, you in the right ways as bitcoin concept in the first place.
Was litteraly the best way to buy bitcoin's for a low fee.
The only thing now is to use Localbitcoins...Talking about the cheapest way.
Hate Coinbase.
The best buy bitcoin is exactly p2p, since bitcoin designed as p2p, decentralized and pseudonym which there's no need 3rd party involved, instead your security is your responsibility.