Usually Hostelworld or booking.com like most. Sometimes after finding the place I want to stay in I'll look directly on their website. When I'm not feeling lazy that is.
I mostly use booking.com - they give discount on some places when you've booked enough stays through them.
But sometimes Hostelworld or directly through the hostel website
I use booking.com and hostelworld as well. Sometimes it's cheaper to book directly from the hostel website. So basically I always use booking and hostelworld to look for great options. Than I check if it's cheaper to book at the original website of the hostel :)
I use both Hostelworld and booking.com 😅 I cross reference the price on both!