Also you frequently get a different price (better) on the app with booking.com and of course if you have VIP status as I do which can give 15-20% off (again not consistent). Have also found recently in some countries (India in particular) Agoda can be cheaper than booking.com despite basically being under the same ownership.
Agree that sometimes cashback sites revert to higher price but then again sometimes they don't. Personally I would suggest shop around, there is no one 'best' place. I haven't had the luck with direct booking that
@lovetravellingoz has but have occasionally found this cheaper, other times have found booking.com, agoda, trip.com cheaper (and sometimes by a lot), even AirBnb on a couple of occasions. If you go by TripAdvisor they can be good at identifying the cheapest (but then sometimes don't know about and hence take account of any status you hold).
I usually check both booking.com and Tripadvisor for reviews (and sometimes Agoda), it can be quite amazing the difference (we know some of the scams being played on TA at least, usually obvious if a hotel has a large number of very positive review from people with only one review).