I've typically used Booking.com since I like how they include all the fees. Lately though I've actually started using Google Maps more since they now integrate hotel pricing (they didn't have it until last year sometime). It's convenient to see it all in one interface geographically.

Once I find a hotel I like, I'll often check the hotel site directly because they can sometimes be cheaper there since they don't have to pay a middleman a fee like Booking.com. Plus, booking directly can also sometimes get you free upgrades compared to booking through a middleman.