https://www.gog.com/games?sort=popularity&page=1&price=u10

What would you pick as must have on this list? Not necessary buy, just "you have to play/have this game, it's great, I bought/played it"...
Not necessary on sale either, just full price worthy.

FTL is my pound-per-pound pick.

Anything worthy? There's a ton of must haves on that list...

I know that. But still, I would like it being listed.

So I've been sucked into this recently, an actual Rougelike that doesn't have a sinfully old interface.

Heroes of M&M III
Curse of Monkey Island
KoTOR
KoTOR II
Secret of Monkey Island
Monkey Island 2
Alladin
FreeSpace 2
Lion King
Descent
Master of Orion


That's just from the first page.
There are 34 pages of games that qualify right now.

Am glad you narrowed your list down to just 34 pages!

I don't know what you like for games or already have but games that i really like for less than 10$ are:

HoMM 3
Wizardry 6-8
Might and Magic 6 Pack Limited Edition and VII
Gothic 1 and 2.
The Temple of Elemental Evil
Age of Wonders 1 and Age of Wonders 2 plus Shadow Magic.
Giant Citizens Kabuto
Anvil of Dawn
Total Annihilation
Return to Castle Wolfenstein
Betrayal at Krondor
Eador Genesis
Empire Earth Gold Edition
Sid Meier's Alpha Centauri
Sid Meier's Civilization III Complete
Sid Meier's Colonization
Sid Meier's Pirates
Thief Gold and Thief 2 Metal age
Icewind Dale 2 Complete
Dungeon Keeper 1 and 2
Hotline Miami
Sacrifice
Caesar 1-4 All games are below 10$
Pharaoh+Cleopatra
XCOM Ufo Defense
Majesty Gold HD
Arcanum
Painkiller Black Edition
The Suffering
Blood:One Unit Whole Blood
Soldier of Fortune
Medal of Honor Allied Assault Warchest
Ultima VII
Lands of Lore 1 and 2.
Psychonauts
Blackwell Bundle and Ephiphany
Gemini Rue
Classic Gabriel Knight adventures, Police Quest collection, King's Quest although varying quality.
Pajama Sam
Call of Cthulhu Dark Corners of the Earth however i don't know the state of this games technical issues.

There's tons of classic games worth buying for less than 10$.

Maybe the D&D archives? Some of them should cost less than 10 $.

Sam and Max is the best game ever.
Emperor's Tomb is the best game ever.
Jedi Academy is the best game ever.
SWAT 4 is the best game ever.
Gothic 2 is the best game ever.
Fallout 2 is the best game ever. So is Fallout. So is Wasteland.
FEAR is the best game ever.
Jagged Alliance 2 is the best game ever.
Evil Genius is the best game ever.
Deus Ex is the best game ever.
System Shock 2 is the best game ever.
Master of Magic is the best game ever.
Arx Fatalis is the best game ever.
Far Cry. Far Cry 2. Colonization. Syberia. Syberia 2. Divine Dininity. Penumbra. Aliens vs Predator. Eschalon. Best games ever.
Ghost Master is the best game ever.
Sands of Time is the BEST game ever.
Carmaggedon and Carmageddon 2 are the best games ever.
Covert action is really the best game ever.
Mount and Blade is the best game ever.
The best game ever is FTL.
World of Goo is the best game ever.
Death to Spies is the BEST best game ever.
And Hard West is the best game ever.

There are other worthy games, though. It is the best game ever.

Adventure: Monkey Island, Indiana Jones and the Fate of Atlantis, Loom
Strategy: Alpha Centauri

I would consider all of them essential in the sense that they were extremely innovative at the time and contributed to shape other references of the genre.

There are so many. Here some of my favorites:

The Cat Lady
The Void
Painkiller + Overdose
Fallout 1+2
Primordia
Risk of Rain
Machinarium
Treasure Adventure World
Shadow Man
Völgarr the Viking
Enter the Gungeon
Saints Row 2

Terraria is insane value for the price. Also Pharaoh if you like city-building/management, along with the other Impressions games. Ditto Rollercoaster Tycoon. Alpha Centauri for 4X. Quest For Glory for adventure-RPG. Far Cry 2 for open-world shooter.

Divine Divinity. Go buy it. Now.

homm series
classic fallout games
icewind dale 2
gothic series (if you can stand the bugs :D)
master of orion series
return to castle wolfenstein
deus ex
system shock 2


there's really a TON :D