There are hundreds of Bitcoin exchanges which anyone can sign up for and start buying Bitcoin quickly and effectively. Personally, I use Gemini or Kraken. Gemini has an excellent interface and low fees, while Kraken offers some of the best crypto assets and is one of the oldest exchanges in existence.
The real question is... What is your favorite Bitcoin exchange?
I like Binance, KuCoin, and Bibox. I have not tried Kraken or Gemini, but perhaps I should check them out.
As for decentralized exchanges, some work needs to be done to make them more user-friendly, but IDEX seems to lead the pack in terms of user-friendliness. However, it lacks support for bitcoin and only allows Ethereum and Ethereum-based tokens. In time there will be a solid decentralized exchange for trading bitcoin.