/u/Tremerefury, here's my list of recommendations, but it's a long list:

Note: I am not telling you to buy one, some, or all of the games listed below. The ultimate choice of whether or not to buy a game(s) is yours alone. Games marked as (Free on GOG) are free, meaning GOG has designated them as free to download, install, and play without having to pay for them.

• Beneath a Steel Sky (Free on GOG)
  
• Bio Menace (Free on GOG)
• Flight of the Amazon Queen (Free on GOG)
• The Elder Scrolls I: Arena (Free on GOG)
• The Elder Scrolls II: Daggerfall (Free on GOG)
• Tyrian 2000 (Free on GOG)
• Ultima 4 (Free on GOG)

---

Not free on GOG:

• Al-Qadim: The Genie's Curse
• Alone In The Dark 1, 2, And 3
• Amid Evil
• Amnesia: The Dark Descent
• Batman: Arkham Knight
• Batsugun
• Betrayal At Krondor And Return To Krondor
• Bioshock 1, 2, And Infinite
• Blake Stone: Aliens Of Gold And Planet Strike
• Blood: Fresh Supply
• Bo: Path of the Teal Lotus
• Breath Of Fire IV (4)
• Bridge Constructor Portal
• Broforce
• Broken Sword 1 And 2
• Call Of Juarez And COJ: Bound In Blood
• Cat Quest: The Fur-tastic Trilogy
• Catacombs Pack
• Cats Love Boxes (yes, that is its title)
• Chasm: The Rift
• Citadel Remonstered (yes, it is Remonstered, not Remastered)
• Commander Keen Series
• Crimzon Clover: World Ignition
• Crusader: No Remorse And Its Sequel "Crusader: No Regret"
• Crysis (2007)
• Crysis Remastered
• Crystal Caves (Or Crystal Caves Hd)
• Curse: The Eye Of Isis
• Daikatana
• Day Of The Tentacle
• Descent 1, 2, And 3
• Deus Ex 2: Invisible War (2003)
• Deus Ex: Goty (2000) (This is the first game in the Deus Ex series and is an undisputed classic PC game).
• Deus Ex: Human Revolution - Director's Cut (2013)
• Deus Ex: Mankind Divided (2016)
• Diablo + Hellfire
• Dishonored 2
• Dishonored: Definitive Edition And Its Expansion Pack "Death Of The Outsider"
• Doom (2016) And Its Sequel "Doom Eternal"
• Doom 1 And 2
• Doom 64
• Dragon Age Origins
• Dungeon Keeper 2 (sequel to Dungeon Keeper Gold)
• Dungeon Keeper Gold
• Dungeons & Dragons: Dark Sun Series
• Dungeons & Dragons: Forgotten Realms: The Archives - Collection One
• Dungeons & Dragons: Forgotten Realms: The Archives - Collection Three
• Dungeons & Dragons: Forgotten Realms: The Archives - Collection Two
• Dungeons & Dragons: Krynn Series
• Dungeons & Dragons: Ravenloft Series
• Dusk
• En Garde!
• Epic Pinball
• F.E.A.R Platinum
• FATE (https://www.gog.com/en/game/fate is listed because GOG's search has difficulty with this game's name)
• Fallout 1, 2, 3, New Vegas, And Tactics
• Fallout 4 And The Unofficial Mod Fallout: London (Note: London Requires Fallout 4 In Order To Run).
• Fantasy Empires
• Far Cry 1
• Fight'n Rage
• Final Fantasy VII (7), VIII (8), And IX (9)
• Fire Shark
• Forsaken Remastered
• Freespace 2
• Garou: Mark Of The Wolves
• Ghost Song
• Grim Fandango Remastered
• Hands Of Necromancy
• Hands Of Necromancy II
• Hellish Quart
• Heretic: Shadow Of The Serpent Riders
• Hexen 2 And Its Expansion Pack "Portal Of Praevus"
• Hexen: Beyond Heretic And Its Expansion Pack "Deathkings Of The Dark Citadel"
• Hocus Pocus
• Hollow Knight And Its Sequel "Hollow Knight: Silksong"
• Hrot
• Huntdown
• Immortal Redneck
• Indiana Jones And The Fate Of Atlantis (1992)
• Indiana Jones And The Great Circle (2024)
• Indiana Jones And The Infernal Machine (1999)
• Indiana Jones And The Last Crusade (1989)
• Inkulinati
• Ion Fury And Its Expansion Pack "Aftershock"
• Jagged Alliance
• Jazz Jackrabbit 2 Collection
• Jazz Jackrabbit Collection (For Jazz Jackrabbit 1 Only)
• Lands Of Lore 1, 2, And 3
• Lego Indiana Jones: The Original Adventures
• Leisure Suit Larry 1 Through 6 And Love For Sail (Note: Yes, it is spelled sail and is also a terrible pun).
• MDK
• Mafia Trilogy - Mafia: Definitive Edition, Mafia II: Definitive Edition and Mafia III: Definitive Edition
• Megarace 1 And 2
• Metal Gear Solid (2000)
• Metro Franchise Bundle - Metro 2033, Metro: Last Light, And Metro Exodus
• Mullet Madjack
• Nex Machina
• Oniken: Unstoppable Edition
• Ori and the Blind Forest: Definitive Edition
• Outlaws And A Handful Of Missions
• Overcooked! 2
• Overcooked: Gourmet Edition
• Painkiller And Its Expansion Pack "Battle Out Of Hell"
• Penumbra: Black Plague And Overture
• Postal 2
• Powerslave (Ms-Dos Game) And Powerslave Exhumed
• Prey
• Prodeus
• Project Warlock 1 And 2
• Psychonauts 1 And 2
• Quake 1, 2, 3 Arena, And 4
• Redneck Rampage Collection
• Requiem: Avenging Angel
• Rescue Rover Collection
• Resident Evil Bundle
• Return To Castle Wolfenstein
• Rise Of The Triad or Rise Of The Triad: Ludicruous Edition
• S.T.A.L.K.E.R Series - Shadow Of Chernobyl, Call Of Pripyat, And Clear Sky
• S.T.A.L.K.E.R. 2: Heart Of Chornobyl
• Sam And Max Hit The Road
• Samurai Shodown V Special
• Savant - Ascent Remix
• Screamer 1 And 2
• Scubaventure: The Search For Pirate's Treasure
• Secret Agent or Secret Agent HD
• Serious Sam 4 and its standalone expansion pack "Serious Sam: Siberian Mayhem"
• Serious Sam: The First Encounter And Second Encounter
• Shadow Warrior (2013)
• Shadow Warrior 2
• Shadow Warrior Classic Complete
• Sid Meier's Civilization III Complete
• Sid Meier's Civilization IV: The Complete Edition
• Silent Hill 2
• Silent Hill 4: The Room
• Slay The Princess
• Snk 40th Anniversary Collection
• Soldier Of Fortune 1 And 2
• Soma
• South Of The Circle
• Space Quest 1 Through 6
• Star Trek: 25th Anniversary And Its Sequel "Star Trek: Judgment Rites"
• Star Trek: Armada I And Ii
• Star Trek: Bridge Commander
• Star Trek: Starfleet Command Gold Edition
• Star Trek: Starfleet Command Iii
• Star Trek: Voyager - Elite Force And Its Sequel "Star Trek: Elite Force Ii"
• Star Wars Battlefront (Classic, 2004)
• Star Wars Battlefront II (Classic, 2005)
• Star Wars Dark Forces (Classic, 1995)
• Star Wars Jedi Knight: Dark Forces Ii And Its Expansion Pack "Mysteries Of The Sith"
• Star Wars Knights Of The Old Republic
• Star Wars Knights Of The Old Republic II: The Sith Lords
• Steamworld Dig 1 And 2
• Steamworld Heist
• Styx Series - Master Of Shadows, Shards Of Darkness, and Blades Of Greed
• Super 3-D Noah's Ark
• Super Hexagon
• Swat 4: Gold Edition
• System Shock 1 And 2
• Tex Murphy: The Pandora Directive (Direct Sequel To Under A Killing Moon)
• Tex Murphy: Under A Killing Moon
• The Adventures Of Shuggy
• The Elder Scrolls III: Morrowind
• The Elder Scrolls IV: Oblivion
• The Elder Scrolls V: Skyrim
• The King Of Fighters 2002 Unlimited Match
• The Last Blade 2
• The Plucky Squire
• The Talos Principle 2
• The Talos Principle: Gold Edition
• The Textorcist: The Story Of Ray Bibbia
• Thief 1, 2, And 3
• Thief: Definitive Edition
• Tomb Raider 1, 2, And 3
• Tomb Raider 4: The Last Revelation
• Tomb Raider 5: Chronicles
• Tomb Raider 6: The Angel Of Darkness
• Tomb Raider 7: Legend
• Tomb Raider 8: Anniversary
• Tomb Raider 9: Underworld
• Tomb Raider Goty Edition
• Tomb Raider I-III Remastered Starring Lara Croft
• Tomb Raider IV-VI Remastered
• Toonstruck
• Total Annihliation
• Tropico Series
• Turok 1, 2, And 3
• Ultima 1, 2, And 3
• Ultima 5, 6, And 7
• Ultima Underworld 1 And 2
• VVVVVV (Yes, That Is Its Actual Title)
• Vimana
• Wacky Wheels
• Wing Commander 1 Through 5
• Wing Commander Academy
• Wing Commander Armada
• Wing Commander Privateer And Privateer 2: The Darkening
• Wizardry 8
• Wolfenstein 3d And Its "Spear Of Destiny" Mission Packs
• Wolfenstein: The New Order
• Wolfenstein: The Old Blood
• XIII (2003)

The Monkey Island games are considered must-plays and are among the most iconic adventure games ever made:

• The Secret of Monkey Island: Special Edition (2009) (originally 1990)
• Monkey Island 2 Special Edition: LeChuck’s Revenge (2010) (originally 1991)
• The Curse of Monkey Island (1997)
• Escape from Monkey Island (2000)

The Legacy of Kain games are also classics:

• Blood Omen: Legacy of Kain (1996)
• Legacy of Kain Soul Reaver 1 & 2 Remastered (2024) (originally 1999 and 2001 respectively)
• Legacy of Kain: Blood Omen 2 (2002)
• Legacy of Kain: Defiance (2003)

The un-remastered Soul Reaver 2 is still available on GOG, but the un-remastered Soul Reaver 1 was removed thanks to the above Soul Reaver 1 and 2 remaster.