Amazon's the obvious answer here, and for most people it'll be the right one. Their platform's built for Kindle, so the file formatting and delivery is seamless. You buy it, it's on your device seconds later. Prices fluctuate a lot though, so it's worth checking if there's a deal on. Sometimes the paperback's cheaper, which feels weird.
One thing that bugs me is you're kinda locked into their ecosystem. If you later get a Kobo or something, you can't transfer it. I've had that happen with a few books I bought early on. Still, for sheer convenience and making sure you get a proper, official copy, that's your spot.