This seemed tricky at first, but actually the answer is easy: Wolfenstein: Enemy Territory. I remember that game being so fun some 20 years ago, and every time I've come back to it, it's been just as fun, if not more (assuming I can evade the nonsensical mods and find human players). Other notable mentions include FreeSpace 2 and the Total War series (not going to bother trying to pick just one for an honorable mention).

This.

No One Lives Forever 2
Sam & Max Hit the Road
Thief: The Dark Project
The Longest Journey
Warcraft 3
World of Warcraft
World in Conflict
Starcraft 1 + 2
Crysis Warhead
Links LS series
Anstoss 2 + 3
Civilization 4 + 5
Europa Universalis series
Ultima 7 - 9
Toonstruck
The Curse of Monkey Island
Zeus: Master of Olympus
Aquanox 1 + 2
Call of Cthulhu 1: Shadow of the Comet
Comanche series
Conflict Freespace
Dungeon Siege 1 + 2
Sacred
Ankh 1 - 3
Jack Keane
Heavy Metal F.A.K.K. 2
Wizardry 8
Pharaoh
The Witcher 1
Fallout 1 + 2
Icewind Dale 2
Tex Murphy series (Under a Killing Moon, Pandora Directive, Overseer)
A Vampyre Story
Black Mirror 1 - 3
Riddle of Master Lu
Desordre

Now this one I am very keen to check out!

I couldn't play it back when it came out as my PC was waaay too weak. I ended up playing Crysis 2 and 3 on my Xbox 360, then Crysis 1 Remastered when it came out on Switch in 2020.

I enjoyed them all, so Warhead is on my wishlist for sure, thanks for reminding me!

I mean most of the big PC games have gotten console ports eventually. One that I believe is still exclusive is Vampire the Masquerade: Bloodlines, one of my favorite RPGs of all time with a truly immersive World. I also think Civilization IV is a PC exclusive, great game!
And since you only specified they couldn't be available on "consoles", I take it that it is fair game to include games that are available on iOS/Android, which means Her Story qualifies, which is one of the most unique pieces of interactive storytelling I have experienced.
Papers Please did receive a Vita port, so I guess it doesn't qualify, it was perfect for PC though.

Honorable mention: Medal of Honor: Allied Assault.

Shoutout to the REAL best PC exclusive!

World of warcraft or Old school runescape. Great MMO games that take up your time.

I've played both Crysis, Warhead, and Crysis 2, and if I recall correctly, I liked Warhead the most. I didn't really like Crysis 2 at all, but I did the original and Warhead. Warhead isn't perfect, but if my memory serves me right, I think it had better pacing/flow than the original, probably also owing to level design.

I am a strategy fanatic... And been a PC gamer for over 30 years... So games like StarCraft I have been playing almost daily since 1998. (27~ years)

1) StarCraft.
2) Age of Empires.
3) Battlezone 2.
4) Dungeon Keeper 2.
5) Master of Orion 2.
6) Alpha Centauri.
7) Homeworld 2.
8) Sacrifice.
9) Command and Conquer: Red Alert 2.
10) Heroes of Might and Magic 2/3.

Have you ever tried Paradox's grand strategy games? A lot of people seem to love them, and based on my limited experience (basically Crusader Kings II/III), no wonder.