https://www.instant-gaming.com/
(Never had single issue in almost 5 years)

https://www.cdkeys.com/
(Often a tiny bit cheaper than instant-gaming, plus you can get 5% off by generating a voucher code here by inputting some random email adress, and then put the code in before you purchase https://www.facebook.com/cdkeyscom/app/393344207416996/?ref=page_internal )

http://www.gamingdragons.com
(Isnt up to date with the latest games, but they can have some pretty good deals at times)