Generally speaking, the fastest way to charge your iPhone is via an iPad charger. Yes, an iPad charger. iPad chargers are 12 watts, while the iPhone charger is only 5 watts. More watts translates to less charging time. By the way, a USB port on your Mac is 2.5 watts, so charging your iPhone by plugging it into your Mac is really a bad idea. It’s convenient, but it just takes too long.
UPDATE: the iPhone 11 Pro and 11 Pro Max come with 18 watt chargers in the box.
The bad news: iPhones don’t come with iPad chargers. The good news: you can get an iPad charger at Amazon (or at an Apple Store). Get the genuine Apple one.
Strictly speaking, there’s an even faster way to charge your iPhone, but only if your iPhone is an iPhone 8, or an 8 plus, or an X, or an Xr, or an Xs, or an Xs Max, or an iPhone 11, 11 Pro, or 11 Pro Max. If you have one of those iPhones you can use the Apple 30W USB-C power adapter. You’ll also need a USB-C to Lightning cable. Bonus: you can use the 30W USB-C charger with a lot of iPads too. See the product page for more details. Order it straight from Apple’s site via those links.