What is the best hotel booking site?

The best prices of any online booking site.
Good customer service when something goes wrong.
Be aware that there are different types of reservations: most can be canceled with no charge, but some have cancellation fees or charge the full amount upon booking with no ability to change or cancel.
Booking.com’s “Verified Reviews” (only confirmed guests can leave a review) are more reliable than what you’ll find on Tripadvisor but fake reviews are still possible. Be sure to read as many reviews as possible.
The maps on Booking.com are better than other sites but they can still be incorrect. If precise location is important then double check on Google Maps.

Booking.com has a super handy calendar view for all the hotels you’ve booked. Now you don’t have to search through your emails to find out where you’re staying on your last night in Paris – it’s all right there in your “Bookings” tab. (This sounds like a minor deal, but it really makes a big difference.)
Reviews on Booking.com are from verified guests (people that actually booked a room through the website at that hotel). Every hotel is graded on friendliness, comfort, location, facilities, staff, value for money, and wifi-quality. I have found a score of 8.8 and above means the hotel will be excellent (pretty much 5 star or exceptional value for money). 8.0 and above is basically a 4 star. 7.0 and above is a good quality 3 star hotel. I never book anything below a score of 7.
Booking.com has the clearest descriptions of the idfferent rooms. Though it’s still far from perfect so additional research is often required to be sure you’re getting the exact room you want.
Booking.com has nice large pictures so you can see what the hotel is like. Also has lots of photos of individual suites and room types (but you need to enter potential dates to see these room pictures).