For hotel bookings I use Agoda for most of my reservations. Here is why I use Agoda:
Lowest price guarantee
Agoda have a best price guarantee so they will match any booking offered by another site for the same hotel and dates.
Search for hotels, hostels, and guesthouses
Agoda lists hotels, hostels, and guesthouses in their search results, so I no longer look for hostels on the hostel booking sites.
Pricing per room, not per bed
One of the frustrating features of the hostel booking sites is how they list private rooms per bed. They don’t reveal how many beds are in the room (that you have to pay for) until you click the link. Agoda list the price of the room, which is especially useful for solo travel bookings.
User-friendly user interface
Search for hotels by price, star rating, location within a city, and specific features.
Agoda have a loyalty program which lets you accrue points with every booking, which can be used as credit for future booking.
Billing in your currency
Bookings can be made in your selected currency which is useful for budgeting and avoids international currency fees on your credit card (eg I am charged 3% on international transactions on my Australian credit card).
In addition to lowest prices, Agoda also have daily deals. For example I found a deal on a 4* hotel in Hong Kong