Try to avoid any credit card/debit card transactions, because these have the most spread and fees when you're transacting. If you have funds in your bank account, the best rates you're going to get is going to come from either cash deposit or bank transfer as a form of payment, not a card payment.
The specific exchange you want to use will depend on the country you're from, and what you need in terms of your volume, demand for convenience/best rates, etc. But regulated, licensed exchanges generally are much more liquid, and will give you the best rates regardless of your country. The only downside is that you need to provide KYC, which can be a very tiresome process to go through. These include exchanges like Kraken, Coinbase Pro, etc.
If you aren't comfortable with providing KYC, I suggest going with p2p exchanges like LBC or Paxful if you want convenience and decent rates. You're protected at all times by escrow, and there is a variety of payment methods.
Wow, thanks for such big answer.