• https://www.skyscanner.net/ - Seems to be the best overall site for buying plane tickets. Lots of good search features, and also shows ticket prices on many other sites as well. No rewards programme, unfortunately, but you can take advantage of their search features to filter to airlines which you have a rewards account with. I would recommend choosing an airline in a big alliance (such as One World), and sticking to airlines within that alliance, even if tickets are slightly more expensive sometimes. You will save more in the long run, as you can use your points to claim free flights, hotels, upgrades, and even free food at airports and on flights.
• https://www.kiwi.com/en/ - Not as many flights as Skyscanner, but has a cool feature where you can set your destination to "anywhere", and an interactive map will pop up - so if you're looking for a cheap adventure but aren't sure where, this site is cool. Skyscanner also has a feature to set the destination as "anywhere", but it's not as good as Kiwi's.
• https://hopper.com/ - Best app/site for hotels. They also sell plane tickets, but they don't seem to be that cheap most of the time (worth checking out though). They have a pretty nice rewards programme to get money off as well. Customer support has been very helpful and friendly on several occasions. They have often contacted hotels themselves on my behalf to request a refund for me, and top-class customer service. 10/10
• https://booking.com/ - Tried and tested, a trustworthy hotel booking site, always worth checking hotel prices on here. Not worth booking flights on here though. Reward programme is not bad, haven't tested it out thoroughly yet. Unlike Hopper, customers are generally expected to contact the hotels directly, so customer service is not as good - but haven't had any bad experiences yet.
• https://hotels.com/ - Similar to booking.com, a mainstream tried and tested hotel booking site. There is a reward programme, but really not that good - you get the 11th night free and a 10% discount on some hotels, but this is definitely the worst out of the three hotel sites here. I would only recommend using this site if you checked the other sites and they have no availability for your chosen hotel/location, or it is cheaper for some reason. However, even if sometimes hotels are slightly more expensive on Hopper (which isn't usually the case, but if they do happen to be), it is probably still worth booking through them for their rewards programme. You will save more in the long run - and also their top-tier customer support is there if anything does go wrong.