Good Steam games for someone new to PC gaming

Maybe The Stanley Parable? Used that game to introduce my father to the idea of computer games

I would recommend the Portal series, they are puzzle games but their not very complicated puzzle games and both have great story! If you want you can even skip portal 1 if it's too old for you :)

Depends what he would enjoy...
I wouldnt suggest starting from brand new AAA titles.

Let him taste some old classics first, Diablo 2? Warcraft 3? Half Life Series? Age of Empires? etc etc

Telltale Games
Portal series
Half Life series
Left 4 Dead 2
Rocket League
Counterstrike
Ori and the Blind Forest
To the Moon
South Park Stick of Truth
Batman Games
Binding of Isaac

I can go on

Bioshock trilogy (the old ones will be remastered)
Dragons Age (origins was the best, but Inquisition is great aswell)
Fallout 4
Dying Light
Just cause 3

Deus Ex
Deus Ex Human Revolution (this is the best game franchise I've ever played, in my opinion)

Mass Effect 1 and 2 (and 3 is available on Origin)
The Walking Dead (pretty damn easy to get into)
Tales from The Borderlands (pretty damn easy to get into)
Tomb Raider (because it feels more like a movie than a game)
Rise of The Tomb Raider
The Witcher games
Portal games (easy to get into)
Skyrim
Bioshock games (Infinite is kinda easier than the others)

maybe something from the borderlands and or farcry franchises

I sense a rather worrying lack of Bad Rats in your list.

In all seriousness, I think you're kind of pushing your favorites onto him. I'd start him with one thing of everything, and let him go his own path from that. Of course, if he asked me. Otherwise, I don't think he'd have much trouble "starting" with pc gaming.

If he's new to PC gaming, get some co-op games and play games with him ...

Portal 2, L4D, etc

Why not suggest something that many of us enjoyed when we were first starting, like Battlefront 2 :)

Metro 2033 and last light without a doubt.
Fallout 3 or Nev vegas for the modding part.
Tomb Raider 2013 and Rise of the tomb raider i guess.
Mad Max for that awesome optimization.
Wolfenstein the new order and old blood.

Orange Box

And do them a favour by not telling them to play half a dozen big open-world games, or acquire lots of games they won't play in general.

Dishonored ಠ_ಠ

Not seeing Dark Souls here. I know the PC Port was underwhelming, but still totes worth it with a controller.

All the fallout games and Elder Scroll games are great on pc and have tons of mods he can mess with after he is used to things :)

Portal
Civilization
XCOM

Something I noticed with your suggestions: Although they all are pretty good games, they also are pretty long. I don't want to know how long it takes to beat just your essentials ;). Additionally they share several characteristics. Big worlds, where you are free to roam and do stuff. Maybe add some more variety. In case he is a console player you maybe also want to showcase some strong points of PC gaming instead of giving him games that got optimized for console controls (unless you want to show off the better graphics a PC can provide).

Portal. Can run in a lot of different systems, and a pretty casual game.

Drop something indie in the list: Bastion, Surgeon Sim and Trine also suggest him to watch Indie Game The Movie.

A few tycoon games, like Rollercoaster? :)

And, of course, a gaming pc library ain't complete without a bunch of random bundles! ^^

If you're gonan get into PC, you try good exclusives. Stuff and genres you cant get on consoles
S.T.A.L.K.E.R. series
ANNO 1404
Company of Heroes
Age of Empires 2
Mount and Blade: Warband
CS:GO (if he hates himself)
Red Orchestra 2/Rising Storm
RUST
ArmA 3
Hard Reset
Killing Floor
Modded Fallout: New Vegas or TES V:Skyrim
Insurgency
Painkiller

And a never ending list of indie games, but a few would be:
The Binding of Isaac
Hotline Miami
FTL
Terraria
Door Kickers
Nuclear Throne
Stanley Parable
La-Mulana
Spelunky
Cave Story

Valve Complete Pack, just wait for next sale

Portal series!

All the games you stated are excellent games. However, I would suggest adding some strategy games as well. What comes to mind:
Civilization 5 (Civ 6 is coming soon tho)
Cities: Skylines
XCOM: Enemy Unknown / XCOM 2
FTL
R.U.S.E.

Also, all the essentials you have given are massive open world games that you could easily sink in 60+ hours. I would recommend spacing them out with less intimidating games, to avoid burnout.

Euro Truck Simulator 2 with Multiplayer
American Truck Simulator also