I add all the games I want to Razer Cortex wishlist, and it says were they're the cheapest. The Razer Game Store is usually the best deal if it's one of the 4 exclusive games each week. Otherwise using zGold on Gamersgate is a good deal too. If it's not on those stores then Steam itself
there is not THE best site out there, its like with supermarkets, you need to compare - i go mainky gir steam, humble bundles (also monthly), gog and comparing with allkeyshop - but there are shady sellers/methods (paying fees, account sharing, region locks) out there too, so read carefully.
Maybe the most famous is Steam, but you can try with CDKeys which is safe to buy on it, Origin you can buy games or subscribe on they monthly rent, Humble Bundle is good even Nuveem is a good place with pretty good prices.
I always check the games I want to buy on isthereanydeal.com. They are a site aggregator that can compare prices of the same games across different sites. Of course sometimes the same games are listed as different names on different websites but you just need to know what you are searching for! They will even show you the available vouchers for the sites the games are bought from.
I've played a lot of different games but most exciting was playing ar games. I've got new feeling. So I'm sure that everyone should try augmented reality apps for pc at least once. There are a lot of game options like rpg, strategy etc.