You've probably heard of Booking.com
Not to knock them, though, it's a hard problem to solve well from an interface and integrations perspective.
They have a huge inventory of hotels, and they have a functional, fairly responsive search interface that isn't annoying. The last part is important — there's little in terms of popups, ads, intrusive upsells, flashy UI elements, and so on. It's not the most modern, but it's very well optimized towards finding stuff. For example, the path to seeing photos is very short (Hipmunk also does well here).
Since they're very popular, they also have the benefit of having a large repository of reviews, and I find it easier than other sites to glean, from reviews and scores, how good a hotel is compared to others. TripAdvisor comes close, but is too annoying.
Hipmunk is okay, but scores lower on usability for me than Booking.com.
Things like: showing fully booked hotels in results by default, showing me how many people are looking at the booking RIGHT NOW!, popups saying where the last person booked from (that's not a flashy UI element to you?), big bold messages like "Limited supply in Madrid: 3 properties like Happy Hostel are already unavailable on our site! ", "You missed it!" next to rooms that you can't get.
As someone else commented, they AB test the crap out of that website because they know that this stuff gets hotels booked like nobody's business. I put up with it because if you ignore that stuff, the information is well laid out, and the prices and experience is otherwise good. Nowadays I check Booking.com and then go straight to the hotel.
On the other hand, do note that the Booking.com review scale is skewed - it's seems to actually be a 2.5-10 scale. The median rating is something like 8.1, so keep that in mind if you're using the review scores.
Have you tried the new UI?
It may not look like much but they know how to drive their consumers to checkout.
And another plus is that they offer free cancellation up to around 24 to 48 hours before your booking.