Whatever is cheap in the next Steam sale lol
But seriously check out
Mount & Blade (Warband is probably the best bet)
Chivalry
Blizzard RTS games
The Witcher (Witcher 1 is good but long and has some slow sections that drag on so you can skip to 2 if you really want)
Recettear: An Item Shop's Tale/Fortune Summoners (since you seem to be into Japanese RPG games, both are really great and different games)
The Binding of Isaac
Dungeons of Dredmor
Minecraft
Civilization (any is good V is probably the most noob friendly)
Avernum: Escape From the Pit or alternatively Avadon (both are good entry points into Spiderweb software's RPGs)
X-COM: UFO Defence (if you liked XCOM but want something meatier and can stand some of the dated elements it's still one of the best games ever made)
Dawn of War 2
Orcs Must Die! 1 & 2
Crusader Kings 2
that should give you a good sampler of the kinds of things PC gaming has to offer, if you have any more specific requests or criteria then I can suggest more. Tell me what PC games you like/d then that will help as well.
The PC gaming catalog is so large and diverse that everyone's idea of must have games can be wildly different.
@TheVoxelman on twitter
