First recommendation is not to buy anything today. Wait until tomorrow, when the Steam holiday sale begins (and goes through … the second of January, I believe).
If you like Borderlands 2, I recommend the Pre-Sequel. Some say it’s not as good, but I liked it more than 2 - you can really see the developers freed themselves a little more from the typical FPS restraints.
Witcher 3 is good. Witcher 2 is garbage, IMHO. You’re better off either reading a synopsis or, better yet, going through the English translations of the books. They’re in slightly different universes, but there’s not all that much backstory, and it’s pretty easy to get into the game. Don’t play 3 on easy mode (“Just the Story”) - then the fights will just be rote. Play at least on medium (“Blood and Broken Bones”). If you’re an action game savant, step it up to the hardest difficulty (“Death March”).
If you like bullet-hell shooters, and also like rogue-likes, I heartily recommend Enter the Gungeon. Solid mechanics, randomized loot, lots of bullets everywhere that reward quick reflexes and a small amount of strategic thinking.
There have been a lot of good RPGs in the last few years. Dishonored and Dishonored 2 are both excellent. I had a lot of fun with Fallout 4 (though, arguably, Fallout: New Vegas is superior).
A lot of the Final Fantasy games (of Playstation fame) are out on PC now. You can get 1 - 13-2, not to mention the MMO (which is #14). #15 just came out on PS4, but there are rumors of a PC release by 2018.
If you prefer turn-based or isometric RPGs, I second the recommendation for any of the Shadowrun games. Also in a similar vein is Satellite Reign, which is a RTS/RPG hybrid in a cyberpunk setting, similar to the old Syndicate franchise. Sorcery! (sold in separate parts) is getting rave reviews as one of the best RPGs of the year, so definitely worth checking out.
XCOM 2 is a really good turn-based strategy game, though I think it’s hurt by the final mission. Shadow Tactics may be superior, though it’s focus is more on stealth.
If you like action RPGs like Diablo, I recommend Grim Dawn. In the same vein, there’s Path of Exile, but for some reason I bounced off of that one pretty quickly (might be the always-online requirement). **Transistor **is another one that didn’t get a lot of attention. If you missed out on Torchlight, get the sequel - it’s better in a lot of ways.
If you like third-person action games, it seems like almost everyone is playing Grand Theft Auto V these days. It has a good single-player campaign, but most people play it for the multiplayer. It can be difficult to work your way up to be able to afford much, but it’s still a lot of fun.
Other murder simulators like GTA: Saint’s Row (I recommend starting with III, then playing IV), Just Cause (2 or 3 are pretty good, though 2 has a better end-game), Shadows of Mordor (plays merry havoc with Tolkien’s lore, but has a good system for creating bad guys), and the new Hitman. Out of all the games in the Assassin’s Creed franchise, I think 3 and 4 are the best ones.
In a more relaxed vein, the city simulator Cities: Skylines is fantastic. The colony management sim RimWorld isn’t done yet, but is in constant development and plays well, even in alpha. You can make some impressive supply chains in Factorio or Big Pharma.
If you really want to put your graphics card through its paces, walking simulators are very relaxing for you and a little taxing on your system (depending on the game, of course). My two favorites are No Man’s Sky (space) and Abzu (ocean).