There are many favorites of mine but there is one game that truly lived up to the hype for me and many others. I am speaking about Portal 2 and it's teaser we received 6 years ago (jesus, that's long ago, I still remember the hype)
I had a blast replaying it with a controller, it's such a great game and yet another sequel that Valve had nailed perfectly. And besides, Wheatley's accent made me love the British accent even more. Here he is in his glory with his gr8 insults:
As for strategy games I really loved playing the heck out of Red Alert 2 (And it's expansion/dlc Yuri's Revenge) along with Generals and Zero Hour. If you want to try the first Red Alert get yourself
OpenRA, a modernized version of Red Alert.
I've tried many sandbox games, obviously Garry's Mod being one of them and have put a lot of time in just one server, the result when you had lots of freetime (coming for your record, Bolli hehe xd)
. Kerbal Space Program is also great with it's scenery and campaign mode that has finally been released, been playing it for a good amount of time when I cracked it like a pleb.
JUST LOOK HOW HAPPY HE IS
However, there was one sandbox game that stood out to me in my X360 days, it's this beauty that many people didn't like for some reason but I enjoyed it greatly.
FPS should be obvious. CS 1.6, CZ (+Deleted Scenes), CS:S and maybe CS:GO if they wouldn't have ruined it with useless skins. As for a second and third favorite it would be BF3 and Bad Company 2 along with COD MW 1 and 2 being in the third place of this category, special honorable mention to Team Fortress 2, still sometimes playing it since 2009.